     [file gcc-common.texi not included]

Copyright © 1995-2004, Free Software Foundation

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with the Invariant Sections being ``GNU Free Documentation License'', with the Front-Cover Texts being ``GNAT Reference Manual'', and with no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''.

GNAT Reference Manual
*********************

GNAT Reference Manual

GNAT, The GNU Ada 95 Compiler

GCC version version-GCC

Ada Core Technologies, Inc.

About This Guide
****************

This manual contains useful information in writing programs using the GNAT compiler.  It includes information on implementation dependent characteristics of GNAT, including all the information required by Annex M of the standard.

Ada 95 is designed to be highly portable. In general, a program will have the same effect even when compiled by different compilers on different platforms. However, since Ada 95 is designed to be used in a wide variety of applications, it also contains a number of system dependent features to be used in interfacing to the external world.

Note: Any program that makes use of implementation-dependent features may be non-portable.  You should follow good programming practice and isolate and clearly document any sections of your program that make use of these features in a non-portable manner.

What This Reference Manual Contains
===================================

This reference manual contains the following chapters:

  * Implementation Defined Pragmas, lists GNAT implementation-dependent pragmas, which can be used to extend and enhance the functionality of the compiler.

  * Implementation Defined Attributes, lists GNAT implementation-dependent attributes which can be used to extend and enhance the functionality of the compiler.

  * Implementation Advice, provides information on generally desirable behavior which are not requirements that all compilers must follow since it cannot be provided on all systems, or which may be undesirable on some systems.

  * Implementation Defined Characteristics, provides a guide to minimizing implementation dependent features.

  * Intrinsic Subprograms, describes the intrinsic subprograms implemented by GNAT, and how they can be imported into user application programs.

  * Representation Clauses and Pragmas, describes in detail the way that GNAT represents data, and in particular the exact set of representation clauses and pragmas that is accepted.

  * Standard Library Routines, provides a listing of packages and a brief description of the functionality that is provided by Ada's extensive set of standard library routines as implemented by GNAT.

  * The Implementation of Standard I/O, details how the GNAT implementation of the input-output facilities.

  * The GNAT Library, is a catalog of packages that complement the Ada predefined library.

  * Interfacing to Other Languages, describes how programs written in Ada using GNAT can be interfaced to other programming languages.

     Specialized Needs Annexes, describes the GNAT implementation of all of the specialized needs annexes.

  * Implementation of Specific Ada Features, discusses issues related to GNAT's implementation of machine code insertions, tasking, and several other features.

  * Project File Reference, presents the syntax and semantics of project files.

  * Obsolescent Features documents implementation dependent features, including pragmas and attributes, which are considered obsolescent, since there are other preferred ways of achieving the same results. These obsolescent forms are retained for backwards compatibilty.

This reference manual assumes that you are familiar with Ada 95 language, as described in the International Standard ANSI/ISO/IEC-8652:1995, Jan 1995.

Conventions
===========

Following are examples of the typographical and graphic conventions used in this guide:

  * Functions, utility program names, standard names, and classes.

  * Option flags

  * File Names, button names, and field names.

  * Variables.

  * Emphasis.

  * [optional information or parameters]

  * Examples are described by text

          and then shown this way.

Commands that are entered by the user are preceded in this manual by the characters $  (dollar sign followed by space).  If your system uses this sequence as a prompt, then the commands will appear exactly as you see them in the manual.  If your system uses some other prompt, then the command will appear with the $ replaced by whatever prompt character you are using.

Related Information
===================

See the following documents for further information on GNAT:

  * GNAT User's Guide, which provides information on how to use the GNAT compiler system.

  * Ada 95 Reference Manual, which contains all reference material for the Ada 95 programming language.

  * Ada 95 Annotated Reference Manual, which is an annotated version of the standard reference manual cited above.  The annotations describe detailed aspects of the design decision, and in particular contain useful sections on Ada 83 compatibility.

  * DEC Ada, Technical Overview and Comparison on DIGITAL Platforms, which contains specific information on compatibility between GNAT and DEC Ada 83 systems.

  * DEC Ada, Language Reference Manual, part number AA-PYZAB-TK which describes in detail the pragmas and attributes provided by the DEC Ada 83 compiler system.

Implementation Defined Pragmas
******************************

Ada 95 defines a set of pragmas that can be used to supply additional information to the compiler.  These language defined pragmas are implemented in GNAT and work as described in the Ada 95 Reference Manual.

In addition, Ada 95 allows implementations to define additional pragmas whose meaning is defined by the implementation.  GNAT provides a number of these implementation-dependent pragmas which can be used to extend and enhance the functionality of the compiler.  This section of the GNAT Reference Manual describes these additional pragmas.

Note that any program using these pragmas may not be portable to other compilers (although GNAT implements this set of pragmas on all platforms).  Therefore if portability to other compilers is an important consideration, the use of these pragmas should be minimized.

Pragma Abort_Defer
==================

Syntax:

     pragma Abort_Defer;

This pragma must appear at the start of the statement sequence of a handled sequence of statements (right after the begin).  It has the effect of deferring aborts for the sequence of statements (but not for the declarations or handlers, if any, associated with this statement sequence).

Pragma Ada_83
=============

Syntax:

     pragma Ada_83;

A configuration pragma that establishes Ada 83 mode for the unit to which it applies, regardless of the mode set by the command line switches.  In Ada 83 mode, GNAT attempts to be as compatible with the syntax and semantics of Ada 83, as defined in the original Ada 83 Reference Manual as possible.  In particular, the new Ada 95 keywords are not recognized, optional package bodies are allowed, and generics may name types with unknown discriminants without using the (<>) notation.  In addition, some but not all of the additional restrictions of Ada 83 are enforced.

Ada 83 mode is intended for two purposes.  Firstly, it allows existing legacy Ada 83 code to be compiled and adapted to GNAT with less effort. Secondly, it aids in keeping code backwards compatible with Ada 83. However, there is no guarantee that code that is processed correctly by GNAT in Ada 83 mode will in fact compile and execute with an Ada 83 compiler, since GNAT does not enforce all the additional checks required by Ada 83.

Pragma Ada_95
=============

Syntax:

     pragma Ada_95;

A configuration pragma that establishes Ada 95 mode for the unit to which it applies, regardless of the mode set by the command line switches. This mode is set automatically for the Ada and System packages and their children, so you need not specify it in these contexts.  This pragma is useful when writing a reusable component that itself uses Ada 95 features, but which is intended to be usable from either Ada 83 or Ada 95 programs.

Pragma Annotate
===============

Syntax:

     pragma Annotate (IDENTIFIER {, ARG});

     ARG ::= NAME | EXPRESSION

This pragma is used to annotate programs.  IDENTIFIER identifies the type of annotation.  GNAT verifies this is an identifier, but does not otherwise analyze it.  The ARG argument can be either a string literal or an expression.  String literals are assumed to be of type Standard.String.  Names of entities are simply analyzed as entity names.  All other expressions are analyzed as expressions, and must be unambiguous.

The analyzed pragma is retained in the tree, but not otherwise processed by any part of the GNAT compiler.  This pragma is intended for use by external tools, including ASIS.

Pragma Assert
=============

Syntax:

     pragma Assert (
       boolean_EXPRESSION
       [, static_string_EXPRESSION]);

The effect of this pragma depends on whether the corresponding command line switch is set to activate assertions.  The pragma expands into code equivalent to the following:

     if assertions-enabled then
        if not boolean_EXPRESSION then
           System.Assertions.Raise_Assert_Failure
             (string_EXPRESSION);
        end if;
     end if;

The string argument, if given, is the message that will be associated with the exception occurrence if the exception is raised.  If no second argument is given, the default message is FILE:NNN, where FILE is the name of the source file containing the assert, and NNN is the line number of the assert.  A pragma is not a statement, so if a statement sequence contains nothing but a pragma assert, then a null statement is required in addition, as in:

     ...
     if J > 3 then
        pragma Assert (K > 3, "Bad value for K");
        null;
     end if;

Note that, as with the if statement to which it is equivalent, the type of the expression is either Standard.Boolean, or any type derived from this standard type.

If assertions are disabled (switch -gnata not used), then there is no effect (and in particular, any side effects from the expression are suppressed).  More precisely it is not quite true that the pragma has no effect, since the expression is analyzed, and may cause types to be frozen if they are mentioned here for the first time.

If assertions are enabled, then the given expression is tested, and if it is False then System.Assertions.Raise_Assert_Failure is called which results in the raising of Assert_Failure with the given message.

If the boolean expression has side effects, these side effects will turn on and off with the setting of the assertions mode, resulting in assertions that have an effect on the program.  You should generally avoid side effects in the expression arguments of this pragma.  However, the expressions are analyzed for semantic correctness whether or not assertions are enabled, so turning assertions on and off cannot affect the legality of a program.

Pragma Ast_Entry
================

Syntax:

     pragma AST_Entry (entry_IDENTIFIER);

This pragma is implemented only in the OpenVMS implementation of GNAT.  The argument is the simple name of a single entry; at most one AST_Entry pragma is allowed for any given entry.  This pragma must be used in conjunction with the AST_Entry attribute, and is only allowed after the entry declaration and in the same task type specification or single task as the entry to which it applies.  This pragma specifies that the given entry may be used to handle an OpenVMS asynchronous system trap (AST) resulting from an OpenVMS system service call.  The pragma does not affect normal use of the entry.  For further details on this pragma, see the DEC Ada Language Reference Manual, section 9.12a.

Pragma C_Pass_By_Copy
=====================

Syntax:

     pragma C_Pass_By_Copy
       ([Max_Size =>] static_integer_EXPRESSION);

Normally the default mechanism for passing C convention records to C convention subprograms is to pass them by reference, as suggested by RM B.3(69).  Use the configuration pragma C_Pass_By_Copy to change this default, by requiring that record formal parameters be passed by copy if all of the following conditions are met:

  * The size of the record type does not exceed
     STATIC_INTEGER_EXPRESSION.

  * The record type has Convention C.

  * The formal parameter has this record type, and the subprogram has a foreign (non-Ada) convention.

If these conditions are met the argument is passed by copy, i.e. in a manner consistent with what C expects if the corresponding formal in the C prototype is a struct (rather than a pointer to a struct).

You can also pass records by copy by specifying the convention C_Pass_By_Copy for the record type, or by using the extended Import and Export pragmas, which allow specification of passing mechanisms on a parameter by parameter basis.

Pragma Comment
==============

Syntax:

     pragma Comment (static_string_EXPRESSION);

This is almost identical in effect to pragma Ident.  It allows the placement of a comment into the object file and hence into the executable file if the operating system permits such usage.  The difference is that Comment, unlike Ident, has no limitations on placement of the pragma (it can be placed anywhere in the main source unit), and if more than one pragma is used, all comments are retained.

Pragma Common_Object
====================

Syntax:

     pragma Common_Object (
          [Internal =>] LOCAL_NAME,
       [, [External =>] EXTERNAL_SYMBOL]
       [, [Size     =>] EXTERNAL_SYMBOL] );

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION

This pragma enables the shared use of variables stored in overlaid linker areas corresponding to the use of COMMON in Fortran.  The single object LOCAL_NAME is assigned to the area designated by the EXTERNAL argument. You may define a record to correspond to a series of fields.  The SIZE argument is syntax checked in GNAT, but otherwise ignored.

Common_Object is not supported on all platforms.  If no support is available, then the code generator will issue a message indicating that the necessary attribute for implementation of this pragma is not available.

Pragma Compile_Time_Warning
===========================

Syntax:

     pragma Compile_Time_Warning
              (boolean_EXPRESSION, static_string_EXPRESSION);

This pragma can be used to generate additional compile time warnings. It is particularly useful in generics, where warnings can be issued for specific problematic instantiations. The first parameter is a boolean expression. The pragma is effective only if the value of this expression is known at compile time, and has the value True. The set of expressions whose values are known at compile time includes all static boolean expressions, and also other values which the compiler can determine at compile time (e.g. the size of a record type set by an explicit size representation clause, or the value of a variable which was initialized to a constant and is known not to have been modified). If these conditions are met, a warning message is generated using the value given as the second argument. This string value may contain embedded ASCII.LF characters to break the message into multiple lines.

Pragma Complex_Representation
=============================

Syntax:

     pragma Complex_Representation
             ([Entity =>] LOCAL_NAME);

The ENTITY argument must be the name of a record type which has two fields of the same floating-point type.  The effect of this pragma is to force gcc to use the special internal complex representation form for this record, which may be more efficient.  Note that this may result in the code for this type not conforming to standard ABI (application binary interface) requirements for the handling of record types.  For example, in some environments, there is a requirement for passing records by pointer, and the use of this pragma may result in passing this type in floating-point registers.

Pragma Component_Alignment
==========================

Syntax:

     pragma Component_Alignment (
          [Form =>] ALIGNMENT_CHOICE
       [, [Name =>] type_LOCAL_NAME]);

     ALIGNMENT_CHOICE ::=
       Component_Size
     | Component_Size_4
     | Storage_Unit
     | Default

Specifies the alignment of components in array or record types. The meaning of the FORM argument is as follows:

Component_Size
     Aligns scalar components and subcomponents of the array or record type on boundaries appropriate to their inherent size (naturally aligned).  For example, 1-byte components are aligned on byte boundaries, 2-byte integer components are aligned on 2-byte boundaries, 4-byte integer components are aligned on 4-byte boundaries and so on.  These alignment rules correspond to the normal rules for C compilers on all machines except the VAX.

Component_Size_4
     Naturally aligns components with a size of four or fewer bytes.  Components that are larger than 4 bytes are placed on the next 4-byte boundary.

Storage_Unit
     Specifies that array or record components are byte aligned, i.e. aligned on boundaries determined by the value of the constant System.Storage_Unit.

Default
     Specifies that array or record components are aligned on default boundaries, appropriate to the underlying hardware or operating system or both.  For OpenVMS VAX systems, the Default choice is the same as the Storage_Unit choice (byte alignment).  For all other systems, the Default choice is the same as Component_Size (natural alignment).

If the Name parameter is present, TYPE_LOCAL_NAME must refer to a local record or array type, and the specified alignment choice applies to the specified type.  The use of Component_Alignment together with a pragma Pack causes the Component_Alignment pragma to be ignored.  The use of Component_Alignment together with a record representation clause is only effective for fields not specified by the representation clause.

If the Name parameter is absent, the pragma can be used as either a configuration pragma, in which case it applies to one or more units in accordance with the normal rules for configuration pragmas, or it can be used within a declarative part, in which case it applies to types that are declared within this declarative part, or within any nested scope within this declarative part.  In either case it specifies the alignment to be applied to any record or array type which has otherwise standard representation.

If the alignment for a record or array type is not specified (using pragma Pack, pragma Component_Alignment, or a record rep clause), the GNAT uses the default alignment as described previously.

Pragma Convention_Identifier
============================

Syntax:

     pragma Convention_Identifier (
              [Name =>]       IDENTIFIER,
              [Convention =>] convention_IDENTIFIER);

This pragma provides a mechanism for supplying synonyms for existing convention identifiers. The Name identifier can subsequently be used as a synonym for the given convention in other pragmas (including for example pragma Import or another Convention_Identifier pragma). As an example of the use of this, suppose you had legacy code which used Fortran77 as the identifier for Fortran. Then the pragma:

     pragma Convention_Identifier (Fortran77, Fortran);

would allow the use of the convention identifier Fortran77 in subsequent code, avoiding the need to modify the sources. As another example, you could use this to parametrize convention requirements according to systems. Suppose you needed to use Stdcall on windows systems, and C on some other system, then you could define a convention identifier Library and use a single Convention_Identifier pragma to specify which convention would be used system-wide.

Pragma CPP_Class
================

Syntax:

     pragma CPP_Class ([Entity =>] LOCAL_NAME);

The argument denotes an entity in the current declarative region that is declared as a tagged or untagged record type.  It indicates that the type corresponds to an externally declared C++ class type, and is to be laid out the same way that C++ would lay out the type.

If (and only if) the type is tagged, at least one component in the record must be of type Interfaces.CPP.Vtable_Ptr, corresponding to the C++ Vtable (or Vtables in the case of multiple inheritance) used for dispatching.

Types for which CPP_Class is specified do not have assignment or equality operators defined (such operations can be imported or declared as subprograms as required).  Initialization is allowed only by constructor functions (see pragma CPP_Constructor).

Pragma CPP_Class is intended primarily for automatic generation using an automatic binding generator tool. See Interfacing to C++ for related information.

Pragma CPP_Constructor
======================

Syntax:

     pragma CPP_Constructor ([Entity =>] LOCAL_NAME);

This pragma identifies an imported function (imported in the usual way with pragma Import) as corresponding to a C++ constructor.  The argument is a name that must have been previously mentioned in a pragma Import with Convention = CPP, and must be of one of the following forms:

  * function FNAME return T'Class

  * function FNAME (...) return T'Class

where T is a tagged type to which the pragma CPP_Class applies.

The first form is the default constructor, used when an object of type T is created on the Ada side with no explicit constructor.  Other constructors (including the copy constructor, which is simply a special case of the second form in which the one and only argument is of type T), can only appear in two contexts:

  * On the right side of an initialization of an object of type T.

  * In an extension aggregate for an object of a type derived from T.

Although the constructor is described as a function that returns a value on the Ada side, it is typically a procedure with an extra implicit argument (the object being initialized) at the implementation level.  GNAT issues the appropriate call, whatever it is, to get the object properly initialized.

In the case of derived objects, you may use one of two possible forms for declaring and creating an object:

  * New_Object : Derived_T

  * New_Object : Derived_T := (CONSTRUCTOR-CALL WITH ...)

In the first case the default constructor is called and extension fields if any are initialized according to the default initialization expressions in the Ada declaration.  In the second case, the given constructor is called and the extension aggregate indicates the explicit values of the extension fields.

If no constructors are imported, it is impossible to create any objects on the Ada side.  If no default constructor is imported, only the initialization forms using an explicit call to a constructor are permitted.

Pragma CPP_Constructor is intended primarily for automatic generation using an automatic binding generator tool. See Interfacing to C++ for more related information.

Pragma CPP_Virtual
==================

Syntax:

     pragma CPP_Virtual
          [Entity     =>] ENTITY,
       [, [Vtable_Ptr =>] vtable_ENTITY,]
       [, [Position   =>] static_integer_EXPRESSION]);

This pragma serves the same function as pragma Import in that case of a virtual function imported from C++.  The ENTITY argument must be a primitive subprogram of a tagged type to which pragma CPP_Class applies.  The VTABLE_PTR argument specifies the Vtable_Ptr component which contains the entry for this virtual function.  The POSITION argument is the sequential number counting virtual functions for this Vtable starting at 1.

The Vtable_Ptr and Position arguments may be omitted if there is one Vtable_Ptr present (single inheritance case) and all virtual functions are imported.  In that case the compiler can deduce both these values.

No External_Name or Link_Name arguments are required for a virtual function, since it is always accessed indirectly via the appropriate Vtable entry.

Pragma CPP_Virtual is intended primarily for automatic generation using an automatic binding generator tool. See Interfacing to C++ for related information.

Pragma CPP_Vtable
=================

Syntax:

     pragma CPP_Vtable (
       [Entity      =>] ENTITY,
       [Vtable_Ptr  =>] vtable_ENTITY,
       [Entry_Count =>] static_integer_EXPRESSION);

Given a record to which the pragma CPP_Class applies, this pragma can be specified for each component of type CPP.Interfaces.Vtable_Ptr. ENTITY is the tagged type, VTABLE_PTR is the record field of type Vtable_Ptr, and ENTRY_COUNT is the number of virtual functions on the C++ side.  Not all of these functions need to be imported on the Ada side.

You may omit the CPP_Vtable pragma if there is only one Vtable_Ptr component in the record and all virtual functions are imported on the Ada side (the default value for the entry count in this case is simply the total number of virtual functions).

Pragma CPP_Vtable is intended primarily for automatic generation using an automatic binding generator tool. See Interfacing to C++ for related information.

Pragma Debug
============

Syntax:

     pragma Debug (PROCEDURE_CALL_WITHOUT_SEMICOLON);

     PROCEDURE_CALL_WITHOUT_SEMICOLON ::=
       PROCEDURE_NAME
     | PROCEDURE_PREFIX ACTUAL_PARAMETER_PART

The argument has the syntactic form of an expression, meeting the syntactic requirements for pragmas.

If assertions are not enabled on the command line, this pragma has no effect.  If asserts are enabled, the semantics of the pragma is exactly equivalent to the procedure call statement corresponding to the argument with a terminating semicolon.  Pragmas are permitted in sequences of declarations, so you can use pragma Debug to intersperse calls to debug procedures in the middle of declarations.

Pragma Detect_Blocking
======================

Syntax:

     pragma Detect_Blocking;

This is a configuration pragma that forces the detection of potentially blocking operations within a protected operation, and to raise Program_Error if that happens.

Pragma Elaboration_Checks
=========================

Syntax:

     pragma Elaboration_Checks (Dynamic | Static);

This is a configuration pragma that provides control over the elaboration model used by the compilation affected by the pragma.  If the parameter is Dynamic, then the dynamic elaboration model described in the Ada Reference Manual is used, as though the -gnatE switch had been specified on the command line.  If the parameter is Static, then the default GNAT static model is used.  This configuration pragma overrides the setting of the command line.  For full details on the elaboration models used by the GNAT compiler, see section ``Elaboration Order Handling in GNAT'' in the GNAT User's Guide.

Pragma Eliminate
================

Syntax:

     pragma Eliminate (
         [Unit_Name =>] IDENTIFIER |
                        SELECTED_COMPONENT);

     pragma Eliminate (
         [Unit_Name       =>]  IDENTIFIER |
                               SELECTED_COMPONENT,
         [Entity          =>]  IDENTIFIER |
                               SELECTED_COMPONENT |
                               STRING_LITERAL
         [,OVERLOADING_RESOLUTION]);

     OVERLOADING_RESOLUTION ::= PARAMETER_AND_RESULT_TYPE_PROFILE |
                                SOURCE_LOCATION

     PARAMETER_AND_RESULT_TYPE_PROFILE ::= PROCEDURE_PROFILE |
                                           FUNCTION_PROFILE

     PROCEDURE_PROFILE ::= Parameter_Types => PARAMETER_TYPES

     FUNCTION_PROFILE ::= [Parameter_Types => PARAMETER_TYPES,]
                           Result_Type => result_SUBTYPE_NAME]

     PARAMETER_TYPES ::= (SUBTYPE_NAME {, SUBTYPE_NAME})
     SUBTYPE_NAME    ::= STRING_VALUE

     SOURCE_LOCATION ::= Source_Location => SOURCE_TRACE
     SOURCE_TRACE    ::= STRING_VALUE

     STRING_VALUE ::= STRING_LITERAL {& STRING_LITERAL}

This pragma indicates that the given entity is not used outside the compilation unit it is defined in. The entity must be an explicitly declared subprogram; this includes  generic subprogram instances and subprograms declared in generic package instances.

If the entity to be eliminated is a library level subprogram, then the first form of pragma Eliminate is used with only a single argument. In this form, the Unit_Name argument specifies the name of the library  level unit to be eliminated.

In all other cases, both Unit_Name and Entity arguments are required. If item is an entity of a library package, then the first argument specifies the unit name, and the second argument specifies the particular entity.  If the second argument is in string form, it must correspond to the internal manner in which GNAT stores entity names (see compilation unit Namet in the compiler sources for details).

The remaining parameters (OVERLOADING_RESOLUTION) are optionally used to distinguish between overloaded subprograms. If a pragma does not contain the OVERLOADING_RESOLUTION parameter(s), it is applied to all the overloaded subprograms denoted by the first two parameters.

Use PARAMETER_AND_RESULT_TYPE_PROFILE to specify the profile of the subprogram to be eliminated in a manner similar to that used for the extended Import and Export pragmas, except that the subtype names are always given as strings. At the moment, this form of distinguishing overloaded subprograms is implemented only partially, so we do not recommend using it for practical subprogram elimination.

Note, that in case of a parameterless procedure its profile is represented as Parameter_Types => ("")

Alternatively, the Source_Location parameter is used to specify which overloaded alternative is to be eliminated by pointing to the location of the DEFINING_PROGRAM_UNIT_NAME of this subprogram in the source text. The string literal (or concatenation of string literals) given as SOURCE_TRACE must have the following format:

     SOURCE_TRACE ::= SOURCE_LOCATION{LBRACKET SOURCE_LOCATION RBRACKET}

     LBRACKET ::= [
     RBRACKET ::= ]

     SOURCE_LOCATION ::= FILE_NAME:LINE_NUMBER
     FILE_NAME       ::= STRING_LITERAL
     LINE_NUMBER     ::= DIGIT {DIGIT}

SOURCE_TRACE should be the short name of the source file (with no directory information), and LINE_NUMBER is supposed to point to the line where the defining name of the subprogram is located.

For the subprograms that are not a part of generic instantiations, only one SOURCE_LOCATION is used. If a subprogram is declared in a package instantiation, SOURCE_TRACE contains two SOURCE_LOCATIONs, the first one is the location of the (DEFINING_PROGRAM_UNIT_NAME of the) instantiation, and the second one denotes the declaration of the corresponding subprogram in the generic package. This approach is recursively used to create SOURCE_LOCATIONs in case of nested instantiations.

The effect of the pragma is to allow the compiler to eliminate the code or data associated with the named entity.  Any reference to an eliminated entity outside the compilation unit it is defined in, causes a compile time or link time error.

The intention of pragma Eliminate is to allow a program to be compiled in a system independent manner, with unused entities eliminated, without the requirement of modifying the source text.  Normally the required set of Eliminate pragmas is constructed automatically using the gnatelim tool. Elimination of unused entities local to a compilation unit is automatic, without requiring the use of pragma Eliminate.

Note that the reason this pragma takes string literals where names might be expected is that a pragma Eliminate can appear in a context where the relevant names are not visible.

Note that any change in the source files that includes removing, splitting of adding lines may make the set of Eliminate pragmas using SOURCE_LOCATION parameter illegal.

Pragma Export_Exception
=======================

Syntax:

     pragma Export_Exception (
          [Internal =>] LOCAL_NAME,
       [, [External =>] EXTERNAL_SYMBOL,]
       [, [Form     =>] Ada | VMS]
       [, [Code     =>] static_integer_EXPRESSION]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION

This pragma is implemented only in the OpenVMS implementation of GNAT.  It causes the specified exception to be propagated outside of the Ada program, so that it can be handled by programs written in other OpenVMS languages. This pragma establishes an external name for an Ada exception and makes the name available to the OpenVMS Linker as a global symbol.  For further details on this pragma, see the DEC Ada Language Reference Manual, section 13.9a3.2.

Pragma Export_Function
======================

Syntax:

     pragma Export_Function (
          [Internal         =>] LOCAL_NAME,
       [, [External         =>] EXTERNAL_SYMBOL]
       [, [Parameter_Types  =>] PARAMETER_TYPES]
       [, [Result_Type      =>] result_SUBTYPE_MARK]
       [, [Mechanism        =>] MECHANISM]
       [, [Result_Mechanism =>] MECHANISM_NAME]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION
     | ""

     PARAMETER_TYPES ::=
       null
     | TYPE_DESIGNATOR {, TYPE_DESIGNATOR}

     TYPE_DESIGNATOR ::=
       subtype_NAME
     | subtype_Name ' Access

     MECHANISM ::=
       MECHANISM_NAME
     | (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})

     MECHANISM_ASSOCIATION ::=
       [formal_parameter_NAME =>] MECHANISM_NAME

     MECHANISM_NAME ::=
       Value
     | Reference

Use this pragma to make a function externally callable and optionally provide information on mechanisms to be used for passing parameter and result values.  We recommend, for the purposes of improving portability, this pragma always be used in conjunction with a separate pragma Export, which must precede the pragma Export_Function. GNAT does not require a separate pragma Export, but if none is present, Convention Ada is assumed, which is usually not what is wanted, so it is usually appropriate to use this pragma in conjunction with a Export or Convention pragma that specifies the desired foreign convention. Pragma Export_Function (and Export, if present) must appear in the same declarative region as the function to which they apply.

INTERNAL_NAME must uniquely designate the function to which the pragma applies.  If more than one function name exists of this name in the declarative part you must use the Parameter_Types and Result_Type parameters is mandatory to achieve the required unique designation.  SUBTYPE_ MARKs in these parameters must exactly match the subtypes in the corresponding function specification, using positional notation to match parameters with subtype marks. The form with an 'Access attribute can be used to match an anonymous access parameter.

Note that passing by descriptor is not supported, even on the OpenVMS ports of GNAT.

Special treatment is given if the EXTERNAL is an explicit null string or a static string expressions that evaluates to the null string. In this case, no external name is generated. This form still allows the specification of parameter mechanisms.

Pragma Export_Object
====================

Syntax:

     pragma Export_Object
           [Internal =>] LOCAL_NAME,
        [, [External =>] EXTERNAL_SYMBOL]
        [, [Size     =>] EXTERNAL_SYMBOL]

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION

This pragma designates an object as exported, and apart from the extended rules for external symbols, is identical in effect to the use of the normal Export pragma applied to an object.  You may use a separate Export pragma (and you probably should from the point of view of portability), but it is not required.  SIZE is syntax checked, but otherwise ignored by GNAT.

Pragma Export_Procedure
=======================

Syntax:

     pragma Export_Procedure (
          [Internal        =>] LOCAL_NAME
       [, [External        =>] EXTERNAL_SYMBOL]
       [, [Parameter_Types =>] PARAMETER_TYPES]
       [, [Mechanism       =>] MECHANISM]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION
     | ""

     PARAMETER_TYPES ::=
       null
     | TYPE_DESIGNATOR {, TYPE_DESIGNATOR}

     TYPE_DESIGNATOR ::=
       subtype_NAME
     | subtype_Name ' Access

     MECHANISM ::=
       MECHANISM_NAME
     | (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})

     MECHANISM_ASSOCIATION ::=
       [formal_parameter_NAME =>] MECHANISM_NAME

     MECHANISM_NAME ::=
       Value
     | Reference

This pragma is identical to Export_Function except that it applies to a procedure rather than a function and the parameters Result_Type and Result_Mechanism are not permitted. GNAT does not require a separate pragma Export, but if none is present, Convention Ada is assumed, which is usually not what is wanted, so it is usually appropriate to use this pragma in conjunction with a Export or Convention pragma that specifies the desired foreign convention.

Note that passing by descriptor is not supported, even on the OpenVMS ports of GNAT.

Special treatment is given if the EXTERNAL is an explicit null string or a static string expressions that evaluates to the null string. In this case, no external name is generated. This form still allows the specification of parameter mechanisms.

Pragma Export_Value
===================

Syntax:

     pragma Export_Value (
       [Value     =>] static_integer_EXPRESSION,
       [Link_Name =>] static_string_EXPRESSION);

This pragma serves to export a static integer value for external use. The first argument specifies the value to be exported. The Link_Name argument specifies the symbolic name to be associated with the integer value. This pragma is useful for defining a named static value in Ada that can be referenced in assembly language units to be linked with the application. This pragma is currently supported only for the AAMP target and is ignored for other targets.

Pragma Export_Valued_Procedure
==============================

Syntax:

     pragma Export_Valued_Procedure (
          [Internal        =>] LOCAL_NAME
       [, [External        =>] EXTERNAL_SYMBOL]
       [, [Parameter_Types =>] PARAMETER_TYPES]
       [, [Mechanism       =>] MECHANISM]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION
     | ""

     PARAMETER_TYPES ::=
       null
     | TYPE_DESIGNATOR {, TYPE_DESIGNATOR}

     TYPE_DESIGNATOR ::=
       subtype_NAME
     | subtype_Name ' Access

     MECHANISM ::=
       MECHANISM_NAME
     | (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})

     MECHANISM_ASSOCIATION ::=
       [formal_parameter_NAME =>] MECHANISM_NAME

     MECHANISM_NAME ::=
       Value
     | Reference

This pragma is identical to Export_Procedure except that the first parameter of LOCAL_NAME, which must be present, must be of mode OUT, and externally the subprogram is treated as a function with this parameter as the result of the function.  GNAT provides for this capability to allow the use of OUT and IN OUT parameters in interfacing to external functions (which are not permitted in Ada functions). GNAT does not require a separate pragma Export, but if none is present, Convention Ada is assumed, which is almost certainly not what is wanted since the whole point of this pragma is to interface with foreign language functions, so it is usually appropriate to use this pragma in conjunction with a Export or Convention pragma that specifies the desired foreign convention.

Note that passing by descriptor is not supported, even on the OpenVMS ports of GNAT.

Special treatment is given if the EXTERNAL is an explicit null string or a static string expressions that evaluates to the null string. In this case, no external name is generated. This form still allows the specification of parameter mechanisms.

Pragma Extend_System
====================

Syntax:

     pragma Extend_System ([Name =>] IDENTIFIER);

This pragma is used to provide backwards compatibility with other implementations that extend the facilities of package System.  In GNAT, System contains only the definitions that are present in the Ada 95 RM.  However, other implementations, notably the DEC Ada 83 implementation, provide many extensions to package System.

For each such implementation accommodated by this pragma, GNAT provides a package Aux_XXX, e.g. Aux_DEC for the DEC Ada 83 implementation, which provides the required additional definitions.  You can use this package in two ways.  You can with it in the normal way and access entities either by selection or using a use clause.  In this case no special processing is required.

However, if existing code contains references such as System.XXX where XXX is an entity in the extended definitions provided in package System, you may use this pragma to extend visibility in System in a non-standard way that provides greater compatibility with the existing code.  Pragma Extend_System is a configuration pragma whose single argument is the name of the package containing the extended definition (e.g. Aux_DEC for the DEC Ada case).  A unit compiled under control of this pragma will be processed using special visibility processing that looks in package System.Aux_XXX where Aux_XXX is the pragma argument for any entity referenced in package System, but not found in package System.

You can use this pragma either to access a predefined System extension supplied with the compiler, for example Aux_DEC or you can construct your own extension unit following the above definition.  Note that such a package is a child of System and thus is considered part of the implementation.  To compile it you will have to use the appropriate switch for compiling system units.  See the GNAT User's Guide for details.

Pragma External
===============

Syntax:

     pragma External (
       [   Convention    =>] convention_IDENTIFIER,
       [   Entity        =>] local_NAME
       [, [External_Name =>] static_string_EXPRESSION ]
       [, [Link_Name     =>] static_string_EXPRESSION ]);

This pragma is identical in syntax and semantics to pragma Export as defined in the Ada Reference Manual.  It is provided for compatibility with some Ada 83 compilers that used this pragma for exactly the same purposes as pragma Export before the latter was standardized.

Pragma External_Name_Casing
===========================

Syntax:

     pragma External_Name_Casing (
       Uppercase | Lowercase
       [, Uppercase | Lowercase | As_Is]);

This pragma provides control over the casing of external names associated with Import and Export pragmas.  There are two cases to consider:

Implicit external names
     Implicit external names are derived from identifiers.  The most common case arises when a standard Ada 95 Import or Export pragma is used with only two arguments, as in:

             pragma Import (C, C_Routine);

     Since Ada is a case insensitive language, the spelling of the identifier in the Ada source program does not provide any information on the desired casing of the external name, and so a convention is needed.  In GNAT the default treatment is that such names are converted to all lower case letters.  This corresponds to the normal C style in many environments. The first argument of pragma External_Name_Casing can be used to control this treatment.  If Uppercase is specified, then the name will be forced to all uppercase letters.  If Lowercase is specified, then the normal default of all lower case letters will be used.

     This same implicit treatment is also used in the case of extended DEC Ada 83 compatible Import and Export pragmas where an external name is explicitly specified using an identifier rather than a string.

Explicit external names
     Explicit external names are given as string literals.  The most common case arises when a standard Ada 95 Import or Export pragma is used with three arguments, as in:

          pragma Import (C, C_Routine, "C_routine");

     In this case, the string literal normally provides the exact casing required for the external name.  The second argument of pragma External_Name_Casing may be used to modify this behavior. If Uppercase is specified, then the name will be forced to all uppercase letters.  If Lowercase is specified, then the name will be forced to all lowercase letters.  A specification of As_Is provides the normal default behavior in which the casing is taken from the string provided.

This pragma may appear anywhere that a pragma is valid.  In particular, it can be used as a configuration pragma in the gnat.adc file, in which case it applies to all subsequent compilations, or it can be used as a program unit pragma, in which case it only applies to the current unit, or it can be used more locally to control individual Import/Export pragmas.

It is primarily intended for use with OpenVMS systems, where many compilers convert all symbols to upper case by default.  For interfacing to such compilers (e.g. the DEC C compiler), it may be convenient to use the pragma:

     pragma External_Name_Casing (Uppercase, Uppercase);

to enforce the upper casing of all external symbols.

Pragma Finalize_Storage_Only
============================

Syntax:

     pragma Finalize_Storage_Only (first_subtype_LOCAL_NAME);

This pragma allows the compiler not to emit a Finalize call for objects defined at the library level.  This is mostly useful for types where finalization is only used to deal with storage reclamation since in most environments it is not necessary to reclaim memory just before terminating execution, hence the name.

Pragma Float_Representation
===========================

Syntax:

     pragma Float_Representation (FLOAT_REP);

     FLOAT_REP ::= VAX_Float | IEEE_Float

This pragma allows control over the internal representation chosen for the predefined floating point types declared in the packages Standard and System. On all systems other than OpenVMS, the argument must be IEEE_Float and the pragma has no effect. On OpenVMS, the argument may be VAX_Float to specify the use of the VAX float format for the floating-point types in Standard. This requires that the standard runtime libraries be recompiled. See the description of the GNAT LIBRARY command in the OpenVMS version of the GNAT Users Guide for details on the use of this command.

Pragma Ident
============

Syntax:

     pragma Ident (static_string_EXPRESSION);

This pragma provides a string identification in the generated object file, if the system supports the concept of this kind of identification string. This pragma is allowed only in the outermost declarative part or declarative items of a compilation unit. If more than one Ident pragma is given, only the last one processed is effective. On OpenVMS systems, the effect of the pragma is identical to the effect of the DEC Ada 83 pragma of the same name. Note that in DEC Ada 83, the maximum allowed length is 31 characters, so if it is important to maintain compatibility with this compiler, you should obey this length limit.

Pragma Import_Exception
=======================

Syntax:

     pragma Import_Exception (
          [Internal =>] LOCAL_NAME,
       [, [External =>] EXTERNAL_SYMBOL,]
       [, [Form     =>] Ada | VMS]
       [, [Code     =>] static_integer_EXPRESSION]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION

This pragma is implemented only in the OpenVMS implementation of GNAT. It allows OpenVMS conditions (for example, from OpenVMS system services or other OpenVMS languages) to be propagated to Ada programs as Ada exceptions. The pragma specifies that the exception associated with an exception declaration in an Ada program be defined externally (in non-Ada code). For further details on this pragma, see the DEC Ada Language Reference Manual, section 13.9a.3.1.

Pragma Import_Function
======================

Syntax:

     pragma Import_Function (
          [Internal                 =>] LOCAL_NAME,
       [, [External                 =>] EXTERNAL_SYMBOL]
       [, [Parameter_Types          =>] PARAMETER_TYPES]
       [, [Result_Type              =>] SUBTYPE_MARK]
       [, [Mechanism                =>] MECHANISM]
       [, [Result_Mechanism         =>] MECHANISM_NAME]
       [, [First_Optional_Parameter =>] IDENTIFIER]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION

     PARAMETER_TYPES ::=
       null
     | TYPE_DESIGNATOR {, TYPE_DESIGNATOR}

     TYPE_DESIGNATOR ::=
       subtype_NAME
     | subtype_Name ' Access

     MECHANISM ::=
       MECHANISM_NAME
     | (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})

     MECHANISM_ASSOCIATION ::=
       [formal_parameter_NAME =>] MECHANISM_NAME

     MECHANISM_NAME ::=
       Value
     | Reference
     | Descriptor [([Class =>] CLASS_NAME)]

     CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca

This pragma is used in conjunction with a pragma Import to specify additional information for an imported function.  The pragma Import (or equivalent pragma Interface) must precede the Import_Function pragma and both must appear in the same declarative part as the function specification.

The INTERNAL argument must uniquely designate the function to which the pragma applies.  If more than one function name exists of this name in the declarative part you must use the Parameter_Types and RESULT_TYPE parameters to achieve the required unique designation.  Subtype marks in these parameters must exactly match the subtypes in the corresponding function specification, using positional notation to match parameters with subtype marks. The form with an 'Access attribute can be used to match an anonymous access parameter.

You may optionally use the MECHANISM and RESULT_MECHANISM parameters to specify passing mechanisms for the parameters and result.  If you specify a single mechanism name, it applies to all parameters.  Otherwise you may specify a mechanism on a parameter by parameter basis using either positional or named notation.  If the mechanism is not specified, the default mechanism is used.

Passing by descriptor is supported only on the OpenVMS ports of GNAT.

First_Optional_Parameter applies only to OpenVMS ports of GNAT. It specifies that the designated parameter and all following parameters are optional, meaning that they are not passed at the generated code level (this is distinct from the notion of optional parameters in Ada where the parameters are passed anyway with the designated optional parameters).  All optional parameters must be of mode IN and have default parameter values that are either known at compile time expressions, or uses of the 'Null_Parameter attribute.

Pragma Import_Object
====================

Syntax:

     pragma Import_Object
          [Internal =>] LOCAL_NAME,
       [, [External =>] EXTERNAL_SYMBOL],
       [, [Size     =>] EXTERNAL_SYMBOL]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION

This pragma designates an object as imported, and apart from the extended rules for external symbols, is identical in effect to the use of the normal Import pragma applied to an object.  Unlike the subprogram case, you need not use a separate Import pragma, although you may do so (and probably should do so from a portability point of view).  SIZE is syntax checked, but otherwise ignored by GNAT.

Pragma Import_Procedure
=======================

Syntax:

     pragma Import_Procedure (
          [Internal                 =>] LOCAL_NAME,
       [, [External                 =>] EXTERNAL_SYMBOL]
       [, [Parameter_Types          =>] PARAMETER_TYPES]
       [, [Mechanism                =>] MECHANISM]
       [, [First_Optional_Parameter =>] IDENTIFIER]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION

     PARAMETER_TYPES ::=
       null
     | TYPE_DESIGNATOR {, TYPE_DESIGNATOR}

     TYPE_DESIGNATOR ::=
       subtype_NAME
     | subtype_Name ' Access

     MECHANISM ::=
       MECHANISM_NAME
     | (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})

     MECHANISM_ASSOCIATION ::=
       [formal_parameter_NAME =>] MECHANISM_NAME

     MECHANISM_NAME ::=
       Value
     | Reference
     | Descriptor [([Class =>] CLASS_NAME)]

     CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca

This pragma is identical to Import_Function except that it applies to a procedure rather than a function and the parameters Result_Type and Result_Mechanism are not permitted.

Pragma Import_Valued_Procedure
==============================

Syntax:

     pragma Import_Valued_Procedure (
          [Internal                 =>] LOCAL_NAME,
       [, [External                 =>] EXTERNAL_SYMBOL]
       [, [Parameter_Types          =>] PARAMETER_TYPES]
       [, [Mechanism                =>] MECHANISM]
       [, [First_Optional_Parameter =>] IDENTIFIER]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION

     PARAMETER_TYPES ::=
       null
     | TYPE_DESIGNATOR {, TYPE_DESIGNATOR}

     TYPE_DESIGNATOR ::=
       subtype_NAME
     | subtype_Name ' Access

     MECHANISM ::=
       MECHANISM_NAME
     | (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})

     MECHANISM_ASSOCIATION ::=
       [formal_parameter_NAME =>] MECHANISM_NAME

     MECHANISM_NAME ::=
       Value
     | Reference
     | Descriptor [([Class =>] CLASS_NAME)]

     CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca

This pragma is identical to Import_Procedure except that the first parameter of LOCAL_NAME, which must be present, must be of mode OUT, and externally the subprogram is treated as a function with this parameter as the result of the function.  The purpose of this capability is to allow the use of OUT and IN OUT parameters in interfacing to external functions (which are not permitted in Ada functions).  You may optionally use the Mechanism parameters to specify passing mechanisms for the parameters. If you specify a single mechanism name, it applies to all parameters. Otherwise you may specify a mechanism on a parameter by parameter basis using either positional or named notation.  If the mechanism is not specified, the default mechanism is used.

Note that it is important to use this pragma in conjunction with a separate pragma Import that specifies the desired convention, since otherwise the default convention is Ada, which is almost certainly not what is required.

Pragma Initialize_Scalars
=========================

Syntax:

     pragma Initialize_Scalars;

This pragma is similar to Normalize_Scalars conceptually but has two important differences.  First, there is no requirement for the pragma to be used uniformly in all units of a partition, in particular, it is fine to use this just for some or all of the application units of a partition, without needing to recompile the run-time library.

In the case where some units are compiled with the pragma, and some without, then a declaration of a variable where the type is defined in package Standard or is locally declared will always be subject to initialization, as will any declaration of a scalar variable.  For composite variables, whether the variable is initialized may also depend on whether the package in which the type of the variable is declared is compiled with the pragma.

The other important difference is that you can control the value used for initializing scalar objects.  At bind time, you can select several options for initialization. You can initialize with invalid values (similar to Normalize_Scalars, though for Initialize_Scalars it is not always possible to determine the invalid values in complex cases like signed component fields with non-standard sizes). You can also initialize with high or low values, or with a specified bit pattern.  See the users guide for binder options for specifying these cases.

This means that you can compile a program, and then without having to recompile the program, you can run it with different values being used for initializing otherwise uninitialized values, to test if your program behavior depends on the choice.  Of course the behavior should not change, and if it does, then most likely you have an erroneous reference to an uninitialized value.

It is even possible to change the value at execution time eliminating even the need to rebind with a different switch using an environment variable. See the GNAT users guide for details.

Note that pragma Initialize_Scalars is particularly useful in conjunction with the enhanced validity checking that is now provided in GNAT, which checks for invalid values under more conditions. Using this feature (see description of the -gnatV flag in the users guide) in conjunction with pragma Initialize_Scalars provides a powerful new tool to assist in the detection of problems caused by uninitialized variables.

Note: the use of Initialize_Scalars has a fairly extensive effect on the generated code. This may cause your code to be substantially larger. It may also cause an increase in the amount of stack required, so it is probably a good idea to turn on stack checking (see description of stack checking in the GNAT users guide) when using this pragma.

Pragma Inline_Always
====================

Syntax:

     pragma Inline_Always (NAME [, NAME]);

Similar to pragma Inline except that inlining is not subject to the use of option -gnatn and the inlining happens regardless of whether this option is used.

Pragma Inline_Generic
=====================

Syntax:

     pragma Inline_Generic (generic_package_NAME);

This is implemented for compatibility with DEC Ada 83 and is recognized, but otherwise ignored, by GNAT.  All generic instantiations are inlined by default when using GNAT.

Pragma Interface
================

Syntax:

     pragma Interface (
          [Convention    =>] convention_identifier,
          [Entity =>] local_name
       [, [External_Name =>] static_string_expression],
       [, [Link_Name     =>] static_string_expression]);

This pragma is identical in syntax and semantics to the standard Ada 95 pragma Import.  It is provided for compatibility with Ada 83.  The definition is upwards compatible both with pragma Interface as defined in the Ada 83 Reference Manual, and also with some extended implementations of this pragma in certain Ada 83 implementations.

Pragma Interface_Name
=====================

Syntax:

     pragma Interface_Name (
          [Entity        =>] LOCAL_NAME
       [, [External_Name =>] static_string_EXPRESSION]
       [, [Link_Name     =>] static_string_EXPRESSION]);

This pragma provides an alternative way of specifying the interface name for an interfaced subprogram, and is provided for compatibility with Ada 83 compilers that use the pragma for this purpose.  You must provide at least one of EXTERNAL_NAME or LINK_NAME.

Pragma Interrupt_Handler
========================

Syntax:

     pragma Interrupt_Handler (procedure_LOCAL_NAME);

This program unit pragma is supported for parameterless protected procedures as described in Annex C of the Ada Reference Manual. On the AAMP target the pragma can also be specified for nonprotected parameterless procedures that are declared at the library level (which includes procedures declared at the top level of a library package). In the case of AAMP, when this pragma is applied to a nonprotected procedure, the instruction IERET is generated for returns from the procedure, enabling maskable interrupts, in place of the normal return instruction.

Pragma Interrupt_State
======================

Syntax:

     pragma Interrupt_State (Name => value, State => SYSTEM | RUNTIME | USER);

Normally certain interrupts are reserved to the implementation.  Any attempt to attach an interrupt causes Program_Error to be raised, as described in RM C.3.2(22).  A typical example is the SIGINT interrupt used in many systems for an Ctrl-C interrupt.  Normally this interrupt is reserved to the implementation, so that Ctrl-C can be used to interrupt execution.  Additionally, signals such as SIGSEGV, SIGABRT, SIGFPE and SIGILL are often mapped to specific Ada exceptions, or used to implement run-time functions such as the abort statement and stack overflow checking.

Pragma Interrupt_State provides a general mechanism for overriding such uses of interrupts.  It subsumes the functionality of pragma Unreserve_All_Interrupts.  Pragma Interrupt_State is not available on OS/2, Windows or VMS.  On all other platforms than VxWorks, it applies to signals; on VxWorks, it applies to vectored hardware interrupts and may be used to mark interrupts required by the board support package as reserved.

Interrupts can be in one of three states:

  * System

     The interrupt is reserved (no Ada handler can be installed), and the Ada run-time may not install a handler. As a result you are guaranteed standard system default action if this interrupt is raised.

  * Runtime

     The interrupt is reserved (no Ada handler can be installed). The run time is allowed to install a handler for internal control purposes, but is not required to do so.

  * User

     The interrupt is unreserved.  The user may install a handler to provide some other action.

These states are the allowed values of the State parameter of the pragma.  The Name parameter is a value of the type Ada.Interrupts.Interrupt_ID.  Typically, it is a name declared in Ada.Interrupts.Names.

This is a configuration pragma, and the binder will check that there are no inconsistencies between different units in a partition in how a given interrupt is specified. It may appear anywhere a pragma is legal.

The effect is to move the interrupt to the specified state.

By declaring interrupts to be SYSTEM, you guarantee the standard system action, such as a core dump.

By declaring interrupts to be USER, you guarantee that you can install a handler.

Note that certain signals on many operating systems cannot be caught and handled by applications.  In such cases, the pragma is ignored.  See the operating system documentation, or the value of the array Reserved declared in the specification of package System.OS_Interface.

Overriding the default state of signals used by the Ada runtime may interfere with an application's runtime behavior in the cases of the synchronous signals, and in the case of the signal used to implement the abort statement.

Pragma Keep_Names
=================

Syntax:

     pragma Keep_Names ([On =>] enumeration_first_subtype_LOCAL_NAME);

The LOCAL_NAME argument must refer to an enumeration first subtype in the current declarative part. The effect is to retain the enumeration literal names for use by Image and Value even if a global Discard_Names pragma applies. This is useful when you want to generally suppress enumeration literal names and for example you therefore use a Discard_Names pragma in the gnat.adc file, but you want to retain the names for specific enumeration types.

Pragma License
==============

Syntax:

     pragma License (Unrestricted | GPL | Modified_GPL | Restricted);

This pragma is provided to allow automated checking for appropriate license conditions with respect to the standard and modified GPL.  A pragma License, which is a configuration pragma that typically appears at the start of a source file or in a separate gnat.adc file, specifies the licensing conditions of a unit as follows:

  * Unrestricted This is used for a unit that can be freely used with no license restrictions. Examples of such units are public domain units, and units from the Ada Reference Manual.

  * GPL This is used for a unit that is licensed under the unmodified GPL, and which therefore cannot be with'ed by a restricted unit.

  * Modified_GPL This is used for a unit licensed under the GNAT modified GPL that includes a special exception paragraph that specifically permits the inclusion of the unit in programs without requiring the entire program to be released under the GPL.  This is the license used for the GNAT run-time which ensures that the run-time can be used freely in any program without GPL concerns.

  * Restricted This is used for a unit that is restricted in that it is not permitted to depend on units that are licensed under the GPL.  Typical examples are proprietary code that is to be released under more restrictive license conditions.  Note that restricted units are permitted to with units which are licensed under the modified GPL (this is the whole point of the modified GPL).

Normally a unit with no License pragma is considered to have an unknown license, and no checking is done.  However, standard GNAT headers are recognized, and license information is derived from them as follows.

     A GNAT license header starts with a line containing 78 hyphens.  The following comment text is searched for the appearance of any of the following strings.

     If the string ``GNU General Public License'' is found, then the unit is assumed to have GPL license, unless the string ``As a special exception'' follows, in which case the license is assumed to be modified GPL.

     If one of the strings ``This specification is adapted from the Ada Semantic Interface'' or ``This specification is derived from the Ada Reference Manual'' is found then the unit is assumed to be unrestricted.

These default actions means that a program with a restricted license pragma will automatically get warnings if a GPL unit is inappropriately with'ed.  For example, the program:

     with Sem_Ch3;
     with GNAT.Sockets;
     procedure Secret_Stuff is
       ...
     end Secret_Stuff

if compiled with pragma License (Restricted) in a gnat.adc file will generate the warning:

     1.  with Sem_Ch3;
             |
        >>> license of withed unit "Sem_Ch3" is incompatible

     2.  with GNAT.Sockets;
     3.  procedure Secret_Stuff is

Here we get a warning on Sem_Ch3 since it is part of the GNAT compiler and is licensed under the GPL, but no warning for GNAT.Sockets which is part of the GNAT run time, and is therefore licensed under the modified GPL.

Pragma Link_With
================

Syntax:

     pragma Link_With (static_string_EXPRESSION {,static_string_EXPRESSION});

This pragma is provided for compatibility with certain Ada 83 compilers. It has exactly the same effect as pragma Linker_Options except that spaces occurring within one of the string expressions are treated as separators. For example, in the following case:

     pragma Link_With ("-labc -ldef");

results in passing the strings -labc and -ldef as two separate arguments to the linker. In addition pragma Link_With allows multiple arguments, with the same effect as successive pragmas.

Pragma Linker_Alias
===================

Syntax:

     pragma Linker_Alias (
       [Entity =>] LOCAL_NAME
       [Alias  =>] static_string_EXPRESSION);

This pragma establishes a linker alias for the given named entity.  For further details on the exact effect, consult the GCC manual.

Pragma Linker_Section
=====================

Syntax:

     pragma Linker_Section (
       [Entity  =>] LOCAL_NAME
       [Section =>] static_string_EXPRESSION);

This pragma specifies the name of the linker section for the given entity. For further details on the exact effect, consult the GCC manual.

Pragma Long_Float
=================

Syntax:

     pragma Long_Float (FLOAT_FORMAT);

     FLOAT_FORMAT ::= D_Float | G_Float

This pragma is implemented only in the OpenVMS implementation of GNAT. It allows control over the internal representation chosen for the predefined type Long_Float and for floating point type representations with digits specified in the range 7 through 15. For further details on this pragma, see the DEC Ada Language Reference Manual, section 3.5.7b.  Note that to use this pragma, the standard runtime libraries must be recompiled.  See the description of the GNAT LIBRARY command in the OpenVMS version of the GNAT User's Guide for details on the use of this command.

Pragma Machine_Attribute
========================

Syntax:

     pragma Machine_Attribute (
       [Attribute_Name =>] string_EXPRESSION,
       [Entity         =>] LOCAL_NAME);

Machine dependent attributes can be specified for types and/or declarations.  Currently only subprogram entities are supported.  This pragma is semantically equivalent to __attribute__((STRING_EXPRESSION)) in GNU C, where STRING_EXPRESSION is recognized by the GNU C macros VALID_MACHINE_TYPE_ATTRIBUTE and VALID_MACHINE_DECL_ATTRIBUTE which are defined in the configuration header file tm.h for each machine.  See the GCC manual for further information.

Pragma Main_Storage
===================

Syntax:

     pragma Main_Storage
       (MAIN_STORAGE_OPTION [, MAIN_STORAGE_OPTION]);

     MAIN_STORAGE_OPTION ::=
       [WORKING_STORAGE =>] static_SIMPLE_EXPRESSION
     | [TOP_GUARD       =>] static_SIMPLE_EXPRESSION

This pragma is provided for compatibility with OpenVMS VAX Systems.  It has no effect in GNAT, other than being syntax checked.  Note that the pragma also has no effect in DEC Ada 83 for OpenVMS Alpha Systems.

Pragma No_Return
================

Syntax:

     pragma No_Return (procedure_LOCAL_NAME);

PROCEDURE_LOCAL_NAME must refer to one or more procedure declarations in the current declarative part.  A procedure to which this pragma is applied may not contain any explicit return statements, and also may not contain any implicit return statements from falling off the end of a statement sequence.  One use of this pragma is to identify procedures whose only purpose is to raise an exception.

Another use of this pragma is to suppress incorrect warnings about missing returns in functions, where the last statement of a function statement sequence is a call to such a procedure.

Pragma Normalize_Scalars
========================

Syntax:

     pragma Normalize_Scalars;

This is a language defined pragma which is fully implemented in GNAT.  The effect is to cause all scalar objects that are not otherwise initialized to be initialized.  The initial values are implementation dependent and are as follows:

Standard.Character

     Objects whose root type is Standard.Character are initialized to Character'Last unless the subtype range excludes NUL (in which case NUL is used). This choice will always generate an invalid value if one exists.

Standard.Wide_Character

     Objects whose root type is Standard.Wide_Character are initialized to Wide_Character'Last unless the subtype range excludes NUL (in which case NUL is used). This choice will always generate an invalid value if one exists.

Standard.Wide_Wide_Character

     Objects whose root type is Standard.Wide_Wide_Character are initialized to the invalid value 16#FFFF_FFFF# unless the subtype range excludes NUL (in which case NUL is used). This choice will always generate an invalid value if one exists.

Integer types

     Objects of an integer type are treated differently depending on whether negative values are present in the subtype. If no negative values are present, then all one bits is used as the initial value except in the special case where zero is excluded from the subtype, in which case all zero bits are used. This choice will always generate an invalid value if one exists.

     For subtypes with negative values present, the largest negative number is used, except in the unusual case where this largest negative number is in the subtype, and the largest positive number is not, in which case the largest positive value is used. This choice will always generate an invalid value if one exists.

Floating-Point Types
     Objects of all floating-point types are initialized to all 1-bits. For standard IEEE format, this corresponds to a NaN (not a number) which is indeed an invalid value.

Fixed-Point Types
     Objects of all fixed-point types are treated as described above for integers, with the rules applying to the underlying integer value used to represent the fixed-point value.

Modular types
     Objects of a modular type are initialized to all one bits, except in the special case where zero is excluded from the subtype, in which case all zero bits are used. This choice will always generate an invalid value if one exists.

Enumeration types
     Objects of an enumeration type are initialized to all one-bits, i.e. to the value 2 ** typ'Size - 1 unless the subtype excludes the literal whose Pos value is zero, in which case a code of zero is used. This choice will always generate an invalid value if one exists.

Pragma Obsolescent
==================

Syntax:

     pragma Obsolescent [(static_string_EXPRESSION)];

This pragma must occur immediately following a subprogram declaration. It indicates that the associated function or procedure is considered obsolescent and should not be used. Typically this is used when an API must be modified by eventually removing or modifying existing subprograms. The pragma can be used at an intermediate stage when the subprogram is still present, but will be removed later.

The effect of this pragma is to output a warning message that the subprogram is obsolescent if the appropriate warning option in the compiler is activated. If a parameter is present, then a second warning message is given containing this text.

Pragma Passive
==============

Syntax:

     pragma Passive ([Semaphore | No]);

Syntax checked, but otherwise ignored by GNAT.  This is recognized for compatibility with DEC Ada 83 implementations, where it is used within a task definition to request that a task be made passive.  If the argument Semaphore is present, or the argument is omitted, then DEC Ada 83 treats the pragma as an assertion that the containing task is passive and that optimization of context switch with this task is permitted and desired.  If the argument No is present, the task must not be optimized.  GNAT does not attempt to optimize any tasks in this manner (since protected objects are available in place of passive tasks).

Pragma Polling
==============

Syntax:

     pragma Polling (ON | OFF);

This pragma controls the generation of polling code.  This is normally off. If pragma Polling (ON) is used then periodic calls are generated to the routine Ada.Exceptions.Poll.  This routine is a separate unit in the runtime library, and can be found in file a-excpol.adb.

Pragma Polling can appear as a configuration pragma (for example it can be placed in the gnat.adc file) to enable polling globally, or it can be used in the statement or declaration sequence to control polling more locally.

A call to the polling routine is generated at the start of every loop and at the start of every subprogram call.  This guarantees that the Poll routine is called frequently, and places an upper bound (determined by the complexity of the code) on the period between two Poll calls.

The primary purpose of the polling interface is to enable asynchronous aborts on targets that cannot otherwise support it (for example Windows NT), but it may be used for any other purpose requiring periodic polling. The standard version is null, and can be replaced by a user program.  This will require re-compilation of the Ada.Exceptions package that can be found in files a-except.ads and a-except.adb.

A standard alternative unit (in file 4wexcpol.adb in the standard GNAT distribution) is used to enable the asynchronous abort capability on targets that do not normally support the capability.  The version of Poll in this file makes a call to the appropriate runtime routine to test for an abort condition.

Note that polling can also be enabled by use of the -gnatP switch.  See the GNAT User's Guide for details.

Pragma Profile (Ravenscar)
==========================

Syntax:

     pragma Profile (Ravenscar);

A configuration pragma that establishes the following set of configuration pragmas:

Task_Dispatching_Policy (FIFO_Within_Priorities)
     [RM D.2.2] Tasks are dispatched following a preemptive priority-ordered scheduling policy.

Locking_Policy (Ceiling_Locking)
     [RM D.3] While tasks and interrupts execute a protected action, they inherit the ceiling priority of the corresponding protected object.

plus the following set of restrictions:

Max_Entry_Queue_Length = 1
     Defines the maximum number of calls that are queued on a (protected) entry. Note that this restrictions is checked at run time. Violation of this restriction results in the raising of Program_Error exception at the point of the call. For the Profile (Ravenscar) the value of Max_Entry_Queue_Length is always 1 and hence no task can be queued on a protected entry.

Max_Protected_Entries = 1
     [RM D.7] Specifies the maximum number of entries per protected type. The bounds of every entry family of a protected unit shall be static, or shall be defined by a discriminant of a subtype whose corresponding bound is static. For the Profile (Ravenscar) the value of Max_Protected_Entries is always 1.

Max_Task_Entries = 0
     [RM D.7] Specifies the maximum number of entries per task.  The bounds of every entry family of a task unit shall be static, or shall be defined by a discriminant of a subtype whose corresponding bound is static.  A value of zero indicates that no rendezvous are possible.  For the Profile (Ravenscar), the value of Max_Task_Entries is always 0 (zero).

No_Abort_Statements
     [RM D.7] There are no abort_statements, and there are no calls to Task_Identification.Abort_Task.

No_Asynchronous_Control
     [RM D.7] There are no semantic dependences on the package Asynchronous_Task_Control.

No_Calendar
     There are no semantic dependencies on the package Ada.Calendar.

No_Dynamic_Attachment
     There is no call to any of the operations defined in package Ada.Interrupts (Is_Reserved, Is_Attached, Current_Handler, Attach_Handler, Exchange_Handler, Detach_Handler, and Reference).

No_Dynamic_Priorities
     [RM D.7] There are no semantic dependencies on the package Dynamic_Priorities.

No_Implicit_Heap_Allocations
     [RM D.7] No constructs are allowed to cause implicit heap allocation.

No_Local_Protected_Objects
     Protected objects and access types that designate such objects shall be declared only at library level.

No_Protected_Type_Allocators
     There are no allocators for protected types or types containing protected subcomponents.

No_Relative_Delay
     There are no delay_relative statements.

No_Requeue_Statements
     Requeue statements are not allowed.

No_Select_Statements
     There are no select_statements.

No_Task_Allocators
     [RM D.7] There are no allocators for task types or types containing task subcomponents.

No_Task_Attributes_Package
     There are no semantic dependencies on the Ada.Task_Attributes package.

No_Task_Hierarchy
     [RM D.7] All (non-environment) tasks depend directly on the environment task of the partition.

No_Task_Termination
     Tasks which terminate are erroneous.

Simple_Barriers
     Entry barrier condition expressions shall be either static boolean expressions or boolean objects which are declared in the protected type which contains the entry.

This set of configuration pragmas and restrictions correspond to the definition of the ``Ravenscar Profile'' for limited tasking, devised and published by the International Real-Time Ada Workshop, 1997, and whose most recent description is available at ftp://ftp.openravenscar.org/openravenscar/ravenscar00.pdf.

The original definition of the profile was revised at subsequent IRTAW meetings. It has been included in the ISO Guide for the Use of the Ada Programming Language in High Integrity Systems, and has been approved by ISO/IEC/SC22/WG9 for inclusion in the next revision of the standard. The formal definition given by the Ada Rapporteur Group (ARG) can be found in two Ada Issues (AI-249 and AI-305) available at http://www.ada-auth.org/cgi-bin/cvsweb.cgi/AIs/AI-00249.TXT and http://www.ada-auth.org/cgi-bin/cvsweb.cgi/AIs/AI-00305.TXT respectively.

The above set is a superset of the restrictions provided by pragma Profile (Restricted), it includes six additional restrictions (Simple_Barriers, No_Select_Statements, No_Calendar, No_Implicit_Heap_Allocations, No_Relative_Delay and No_Task_Termination).  This means that pragma Profile (Ravenscar), like the pragma Profile (Restricted), automatically causes the use of a simplified, more efficient version of the tasking run-time system.

Pragma Profile (Restricted)
===========================

Syntax:

     pragma Profile (Restricted);

A configuration pragma that establishes the following set of restrictions:

  * No_Abort_Statements

  * No_Entry_Queue

  * No_Task_Hierarchy

  * No_Task_Allocators

  * No_Dynamic_Priorities

  * No_Terminate_Alternatives

  * No_Dynamic_Attachment

  * No_Protected_Type_Allocators

  * No_Local_Protected_Objects

  * No_Requeue_Statements

  * No_Task_Attributes_Package

  * Max_Asynchronous_Select_Nesting =  0

  * Max_Task_Entries =  0

  * Max_Protected_Entries = 1

  * Max_Select_Alternatives = 0

This set of restrictions causes the automatic selection of a simplified version of the run time that provides improved performance for the limited set of tasking functionality permitted by this set of restrictions.

Pragma Propagate_Exceptions
===========================

Syntax:

     pragma Propagate_Exceptions (subprogram_LOCAL_NAME);

This pragma indicates that the given entity, which is the name of an imported foreign-language subprogram may receive an Ada exception, and that the exception should be propagated.  It is relevant only if zero cost exception handling is in use, and is thus never needed if the alternative longjmp / setjmp implementation of exceptions is used (although it is harmless to use it in such cases).

The implementation of fast exceptions always properly propagates exceptions through Ada code, as described in the Ada Reference Manual. However, this manual is silent about the propagation of exceptions through foreign code.  For example, consider the situation where P1 calls P2, and P2 calls P3, where P1 and P3 are in Ada, but P2 is in C. P3 raises an Ada exception.  The question is whether or not it will be propagated through P2 and can be handled in P1.

For the longjmp / setjmp implementation of exceptions, the answer is always yes.  For some targets on which zero cost exception handling is implemented, the answer is also always yes.  However, there are some targets, notably in the current version all x86 architecture targets, in which the answer is that such propagation does not happen automatically.  If such propagation is required on these targets, it is mandatory to use Propagate_Exceptions to name all foreign language routines through which Ada exceptions may be propagated.

Pragma Psect_Object
===================

Syntax:

     pragma Psect_Object (
          [Internal =>] LOCAL_NAME,
       [, [External =>] EXTERNAL_SYMBOL]
       [, [Size     =>] EXTERNAL_SYMBOL]);

     EXTERNAL_SYMBOL ::=
       IDENTIFIER
     | static_string_EXPRESSION

This pragma is identical in effect to pragma Common_Object.

Pragma Pure_Function
====================

Syntax:

     pragma Pure_Function ([Entity =>] function_LOCAL_NAME);

This pragma appears in the same declarative part as a function declaration (or a set of function declarations if more than one overloaded declaration exists, in which case the pragma applies to all entities).  It specifies that the function Entity is to be considered pure for the purposes of code generation.  This means that the compiler can assume that there are no side effects, and in particular that two calls with identical arguments produce the same result.  It also means that the function can be used in an address clause.

Note that, quite deliberately, there are no static checks to try to ensure that this promise is met, so Pure_Function can be used with functions that are conceptually pure, even if they do modify global variables.  For example, a square root function that is instrumented to count the number of times it is called is still conceptually pure, and can still be optimized, even though it modifies a global variable (the count).  Memo functions are another example (where a table of previous calls is kept and consulted to avoid re-computation).

Note: Most functions in a Pure package are automatically pure, and there is no need to use pragma Pure_Function for such functions.  One exception is any function that has at least one formal of type System.Address or a type derived from it.  Such functions are not considered pure by default, since the compiler assumes that the Address parameter may be functioning as a pointer and that the referenced data may change even if the address value does not. Similarly, imported functions are not considered to be pure by default, since there is no way of checking that they are in fact pure.  The use of pragma Pure_Function for such a function will override these default assumption, and cause the compiler to treat a designated subprogram as pure in these cases.

Note: If pragma Pure_Function is applied to a renamed function, it applies to the underlying renamed function.  This can be used to disambiguate cases of overloading where some but not all functions in a set of overloaded functions are to be designated as pure.

Pragma Restriction_Warnings
===========================

Syntax:

     pragma Restriction_Warnings
       (restriction_IDENTIFIER {, restriction_IDENTIFIER});

This pragma allows a series of restriction identifiers to be specified (the list of allowed identifiers is the same as for pragma Restrictions). For each of these identifiers the compiler checks for violations of the restriction, but generates a warning message rather than an error message if the restriction is violated.

Pragma Source_File_Name
=======================

Syntax:

     pragma Source_File_Name (
       [Unit_Name   =>] unit_NAME,
       Spec_File_Name =>  STRING_LITERAL);

     pragma Source_File_Name (
       [Unit_Name   =>] unit_NAME,
       Body_File_Name =>  STRING_LITERAL);

Use this to override the normal naming convention.  It is a configuration pragma, and so has the usual applicability of configuration pragmas (i.e. it applies to either an entire partition, or to all units in a compilation, or to a single unit, depending on how it is used. UNIT_NAME is mapped to FILE_NAME_LITERAL.  The identifier for the second argument is required, and indicates whether this is the file name for the spec or for the body.

Another form of the Source_File_Name pragma allows the specification of patterns defining alternative file naming schemes to apply to all files.

     pragma Source_File_Name
       (Spec_File_Name => STRING_LITERAL
        [,Casing => CASING_SPEC]
        [,Dot_Replacement => STRING_LITERAL]);

     pragma Source_File_Name
       (Body_File_Name => STRING_LITERAL
        [,Casing => CASING_SPEC]
        [,Dot_Replacement => STRING_LITERAL]);

     pragma Source_File_Name
       (Subunit_File_Name => STRING_LITERAL
        [,Casing => CASING_SPEC]
        [,Dot_Replacement => STRING_LITERAL]);

     CASING_SPEC ::= Lowercase | Uppercase | Mixedcase

The first argument is a pattern that contains a single asterisk indicating the point at which the unit name is to be inserted in the pattern string to form the file name.  The second argument is optional.  If present it specifies the casing of the unit name in the resulting file name string. The default is lower case.  Finally the third argument allows for systematic replacement of any dots in the unit name by the specified string literal.

A pragma Source_File_Name cannot appear after a Pragma Source_File_Name_Project.

For more details on the use of the Source_File_Name pragma, see the sections ``Using Other File Names'' and ``Alternative File Naming Schemes'' in the GNAT User's Guide.

Pragma Source_File_Name_Project
===============================

This pragma has the same syntax and semantics as pragma Source_File_Name. It is only allowed as a stand alone configuration pragma. It cannot appear after a Pragma Source_File_Name, and most importantly, once pragma Source_File_Name_Project appears, no further Source_File_Name pragmas are allowed.

The intention is that Source_File_Name_Project pragmas are always generated by the Project Manager in a manner consistent with the naming specified in a project file, and when naming is controlled in this manner, it is not permissible to attempt to modify this naming scheme using Source_File_Name pragmas (which would not be known to the project manager).

Pragma Source_Reference
=======================

Syntax:

     pragma Source_Reference (INTEGER_LITERAL, STRING_LITERAL);

This pragma must appear as the first line of a source file. INTEGER_LITERAL is the logical line number of the line following the pragma line (for use in error messages and debugging information).  STRING_LITERAL is a static string constant that specifies the file name to be used in error messages and debugging information.  This is most notably used for the output of gnatchop with the -r switch, to make sure that the original unchopped source file is the one referred to.

The second argument must be a string literal, it cannot be a static string expression other than a string literal.  This is because its value is needed for error messages issued by all phases of the compiler.

Pragma Stream_Convert
=====================

Syntax:

     pragma Stream_Convert (
       [Entity =>] type_LOCAL_NAME,
       [Read   =>] function_NAME,
       [Write  =>] function_NAME);

This pragma provides an efficient way of providing stream functions for types defined in packages.  Not only is it simpler to use than declaring the necessary functions with attribute representation clauses, but more significantly, it allows the declaration to made in such a way that the stream packages are not loaded unless they are needed.  The use of the Stream_Convert pragma adds no overhead at all, unless the stream attributes are actually used on the designated type.

The first argument specifies the type for which stream functions are provided.  The second parameter provides a function used to read values of this type.  It must name a function whose argument type may be any subtype, and whose returned type must be the type given as the first argument to the pragma.

The meaning of the READ parameter is that if a stream attribute directly or indirectly specifies reading of the type given as the first parameter, then a value of the type given as the argument to the Read function is read from the stream, and then the Read function is used to convert this to the required target type.

Similarly the WRITE parameter specifies how to treat write attributes that directly or indirectly apply to the type given as the first parameter. It must have an input parameter of the type specified by the first parameter, and the return type must be the same as the input type of the Read function. The effect is to first call the Write function to convert to the given stream type, and then write the result type to the stream.

The Read and Write functions must not be overloaded subprograms.  If necessary renamings can be supplied to meet this requirement. The usage of this attribute is best illustrated by a simple example, taken from the GNAT implementation of package Ada.Strings.Unbounded:

     function To_Unbounded (S : String)
                return Unbounded_String
       renames To_Unbounded_String;

     pragma Stream_Convert
       (Unbounded_String, To_Unbounded, To_String);

The specifications of the referenced functions, as given in the Ada 95 Reference Manual are:

     function To_Unbounded_String (Source : String)
       return Unbounded_String;

     function To_String (Source : Unbounded_String)
       return String;

The effect is that if the value of an unbounded string is written to a stream, then the representation of the item in the stream is in the same format used for Standard.String, and this same representation is expected when a value of this type is read from the stream.

Pragma Style_Checks
===================

Syntax:

     pragma Style_Checks (string_LITERAL | ALL_CHECKS |
                          On | Off [, LOCAL_NAME]);

This pragma is used in conjunction with compiler switches to control the built in style checking provided by GNAT.  The compiler switches, if set, provide an initial setting for the switches, and this pragma may be used to modify these settings, or the settings may be provided entirely by the use of the pragma.  This pragma can be used anywhere that a pragma is legal, including use as a configuration pragma (including use in the gnat.adc file).

The form with a string literal specifies which style options are to be activated.  These are additive, so they apply in addition to any previously set style check options.  The codes for the options are the same as those used in the -gnaty switch to gcc or gnatmake. For example the following two methods can be used to enable layout checking:

  * pragma Style_Checks ("l");

  * gcc -c -gnatyl ...

The form ALL_CHECKS activates all standard checks (its use is equivalent to the use of the gnaty switch with no options.  See GNAT User's Guide for details.

The forms with Off and On can be used to temporarily disable style checks as shown in the following example:

     pragma Style_Checks ("k"); -- requires keywords in lower case
     pragma Style_Checks (Off); -- turn off style checks
     NULL;                      -- this will not generate an error message
     pragma Style_Checks (On);  -- turn style checks back on
     NULL;                      -- this will generate an error message

Finally the two argument form is allowed only if the first argument is On or Off.  The effect is to turn of semantic style checks for the specified entity, as shown in the following example:

     pragma Style_Checks ("r"); -- require consistency of identifier casing
     Arg : Integer;
     Rf1 : Integer := ARG;      -- incorrect, wrong case
     pragma Style_Checks (Off, Arg);
     Rf2 : Integer := ARG;      -- OK, no error

Pragma Subtitle
===============

Syntax:

     pragma Subtitle ([Subtitle =>] STRING_LITERAL);

This pragma is recognized for compatibility with other Ada compilers but is ignored by GNAT.

Pragma Suppress_All
===================

Syntax:

     pragma Suppress_All;

This pragma can only appear immediately following a compilation unit.  The effect is to apply Suppress (All_Checks) to the unit which it follows.  This pragma is implemented for compatibility with DEC Ada 83 usage.  The use of pragma Suppress (All_Checks) as a normal configuration pragma is the preferred usage in GNAT.

Pragma Suppress_Exception_Locations
===================================

Syntax:

     pragma Suppress_Exception_Locations;

In normal mode, a raise statement for an exception by default generates an exception message giving the file name and line number for the location of the raise. This is useful for debugging and logging purposes, but this entails extra space for the strings for the messages. The configuration pragma Suppress_Exception_Locations can be used to suppress the generation of these strings, with the result that space is saved, but the exception message for such raises is null. This configuration pragma may appear in a global configuration pragma file, or in a specific unit as usual. It is not required that this pragma be used consistently within a partition, so it is fine to have some units within a partition compiled with this pragma and others compiled in normal mode without it.

Pragma Suppress_Initialization
==============================

Syntax:

     pragma Suppress_Initialization ([Entity =>] type_Name);

This pragma suppresses any implicit or explicit initialization associated with the given type name for all variables of this type.

Pragma Task_Info
================

Syntax

     pragma Task_Info (EXPRESSION);

This pragma appears within a task definition (like pragma Priority) and applies to the task in which it appears.  The argument must be of type System.Task_Info.Task_Info_Type. The Task_Info pragma provides system dependent control over aspects of tasking implementation, for example, the ability to map tasks to specific processors.  For details on the facilities available for the version of GNAT that you are using, see the documentation in the specification of package System.Task_Info in the runtime library.

Pragma Task_Name
================

Syntax

     pragma Task_Name (string_EXPRESSION);

This pragma appears within a task definition (like pragma Priority) and applies to the task in which it appears.  The argument must be of type String, and provides a name to be used for the task instance when the task is created.  Note that this expression is not required to be static, and in particular, it can contain references to task discriminants.  This facility can be used to provide different names for different tasks as they are created, as illustrated in the example below.

The task name is recorded internally in the run-time structures and is accessible to tools like the debugger.  In addition the routine Ada.Task_Identification.Image will return this string, with a unique task address appended.

     --  Example of the use of pragma Task_Name

     with Ada.Task_Identification;
     use Ada.Task_Identification;
     with Text_IO; use Text_IO;
     procedure t3 is

        type Astring is access String;

        task type Task_Typ (Name : access String) is
           pragma Task_Name (Name.all);
        end Task_Typ;

        task body Task_Typ is
           Nam : constant String := Image (Current_Task);
        begin
           Put_Line ("-->" & Nam (1 .. 14) & "<--");
        end Task_Typ;

        type Ptr_Task is access Task_Typ;
        Task_Var : Ptr_Task;

     begin
        Task_Var :=
          new Task_Typ (new String'("This is task 1"));
        Task_Var :=
          new Task_Typ (new String'("This is task 2"));
     end;

Pragma Task_Storage
===================

Syntax:

     pragma Task_Storage (
       [Task_Type =>] LOCAL_NAME,
       [Top_Guard =>] static_integer_EXPRESSION);

This pragma specifies the length of the guard area for tasks.  The guard area is an additional storage area allocated to a task.  A value of zero means that either no guard area is created or a minimal guard area is created, depending on the target.  This pragma can appear anywhere a Storage_Size attribute definition clause is allowed for a task type.

Pragma Thread_Body
==================

Syntax:

     pragma Thread_Body (
       [Entity =>] LOCAL_NAME,
      [[Secondary_Stack_Size =>] static_integer_EXPRESSION)];

This pragma specifies that the subprogram whose name is given as the Entity argument is a thread body, which will be activated by being called via its Address from foreign code. The purpose is to allow execution and registration of the foreign thread within the Ada run-time system.

See the library unit System.Threads for details on the expansion of a thread body subprogram, including the calls made to subprograms within System.Threads to register the task.  This unit also lists the targets and runtime systems for which this pragma is supported.

A thread body subprogram may not be called directly from Ada code, and it is not permitted to apply the Access (or Unrestricted_Access) attributes to such a subprogram. The only legitimate way of calling such a subprogram is to pass its Address to foreign code and then make the call from the foreign code.

A thread body subprogram may have any parameters, and it may be a function returning a result. The convention of the thread body subprogram may be set in the usual manner using pragma Convention.

The secondary stack size parameter, if given, is used to set the size of secondary stack for the thread. The secondary stack is allocated as a local variable of the expanded thread body subprogram, and thus is allocated out of the main thread stack size. If no secondary stack size parameter is present, the default size (from the declaration in System.Secondary_Stack is used.

Pragma Time_Slice
=================

Syntax:

     pragma Time_Slice (static_duration_EXPRESSION);

For implementations of GNAT on operating systems where it is possible to supply a time slice value, this pragma may be used for this purpose. It is ignored if it is used in a system that does not allow this control, or if it appears in other than the main program unit. Note that the effect of this pragma is identical to the effect of the DEC Ada 83 pragma of the same name when operating under OpenVMS systems.

Pragma Title
============

Syntax:

     pragma Title (TITLING_OPTION [, TITLING OPTION]);

     TITLING_OPTION ::=
       [Title    =>] STRING_LITERAL,
     | [Subtitle =>] STRING_LITERAL

Syntax checked but otherwise ignored by GNAT.  This is a listing control pragma used in DEC Ada 83 implementations to provide a title and/or subtitle for the program listing.  The program listing generated by GNAT does not have titles or subtitles.

Unlike other pragmas, the full flexibility of named notation is allowed for this pragma, i.e. the parameters may be given in any order if named notation is used, and named and positional notation can be mixed following the normal rules for procedure calls in Ada.

Pragma Unchecked_Union
======================

Syntax:

     pragma Unchecked_Union (first_subtype_LOCAL_NAME);

This pragma is used to declare that the specified type should be represented in a manner equivalent to a C union type, and is intended only for use in interfacing with C code that uses union types.  In Ada terms, the named type must obey the following rules:

  * It is a non-tagged non-limited record type.

  * It has a single discrete discriminant with a default value.

  * The component list consists of a single variant part.

  * Each variant has a component list with a single component.

  * No nested variants are allowed.

  * No component has an explicit default value.

  * No component has a non-static constraint.

In addition, given a type that meets the above requirements, the following restrictions apply to its use throughout the program:

  * The discriminant name can be mentioned only in an aggregate.

  * No subtypes may be created of this type.

  * The type may not be constrained by giving a discriminant value.

  * The type cannot be passed as the actual for a generic formal with a discriminant.

Equality and inequality operations on unchecked_unions are not available, since there is no discriminant to compare and the compiler does not even know how many bits to compare.  It is implementation dependent whether this is detected at compile time as an illegality or whether it is undetected and considered to be an erroneous construct.  In GNAT, a direct comparison is illegal, but GNAT does not attempt to catch the composite case (where two composites are compared that contain an unchecked union component), so such comparisons are simply considered erroneous.

The layout of the resulting type corresponds exactly to a C union, where each branch of the union corresponds to a single variant in the Ada record.  The semantics of the Ada program is not changed in any way by the pragma, i.e. provided the above restrictions are followed, and no erroneous incorrect references to fields or erroneous comparisons occur, the semantics is exactly as described by the Ada reference manual. Pragma Suppress (Discriminant_Check) applies implicitly to the type and the default convention is C.

Pragma Unimplemented_Unit
=========================

Syntax:

     pragma Unimplemented_Unit;

If this pragma occurs in a unit that is processed by the compiler, GNAT aborts with the message XXX not implemented, where XXX is the name of the current compilation unit.  This pragma is intended to allow the compiler to handle unimplemented library units in a clean manner.

The abort only happens if code is being generated.  Thus you can use specs of unimplemented packages in syntax or semantic checking mode.

Pragma Universal_Data
=====================

Syntax:

     pragma Universal_Data [(library_unit_Name)];

This pragma is supported only for the AAMP target and is ignored for other targets. The pragma specifies that all library-level objects (Counter 0 data) associated with the library unit are to be accessed and updated using universal addressing (24-bit addresses for AAMP5) rather than the default of 16-bit Data Environment (DENV) addressing. Use of this pragma will generally result in less efficient code for references to global data associated with the library unit, but allows such data to be located anywhere in memory. This pragma is a library unit pragma, but can also be used as a configuration pragma (including use in the gnat.adc file). The functionality of this pragma is also available by applying the -univ switch on the compilations of units where universal addressing of the data is desired.

Pragma Unreferenced
===================

Syntax:

     pragma Unreferenced (local_Name {, local_Name});

This pragma signals that the entities whose names are listed are deliberately not referenced in the current source unit. This suppresses warnings about the entities being unreferenced, and in addition a warning will be generated if one of these entities is in fact referenced in the same unit as the pragma (or in the corresponding body, or one of its subunits).

This is particularly useful for clearly signaling that a particular parameter is not referenced in some particular subprogram implementation and that this is deliberate. It can also be useful in the case of objects declared only for their initialization or finalization side effects.

If local_Name identifies more than one matching homonym in the current scope, then the entity most recently declared is the one to which the pragma applies.

The left hand side of an assignment does not count as a reference for the purpose of this pragma. Thus it is fine to assign to an entity for which pragma Unreferenced is given.

Pragma Unreserve_All_Interrupts
===============================

Syntax:

     pragma Unreserve_All_Interrupts;

Normally certain interrupts are reserved to the implementation.  Any attempt to attach an interrupt causes Program_Error to be raised, as described in RM C.3.2(22).  A typical example is the SIGINT interrupt used in many systems for a Ctrl-C interrupt.  Normally this interrupt is reserved to the implementation, so that Ctrl-C can be used to interrupt execution.

If the pragma Unreserve_All_Interrupts appears anywhere in any unit in a program, then all such interrupts are unreserved.  This allows the program to handle these interrupts, but disables their standard functions.  For example, if this pragma is used, then pressing Ctrl-C will not automatically interrupt execution.  However, a program can then handle the SIGINT interrupt as it chooses.

For a full list of the interrupts handled in a specific implementation, see the source code for the specification of Ada.Interrupts.Names in file a-intnam.ads.  This is a target dependent file that contains the list of interrupts recognized for a given target.  The documentation in this file also specifies what interrupts are affected by the use of the Unreserve_All_Interrupts pragma.

For a more general facility for controlling what interrupts can be handled, see pragma Interrupt_State, which subsumes the functionality of the Unreserve_All_Interrupts pragma.

Pragma Unsuppress
=================

Syntax:

     pragma Unsuppress (IDENTIFIER [, [On =>] NAME]);

This pragma undoes the effect of a previous pragma Suppress.  If there is no corresponding pragma Suppress in effect, it has no effect.  The range of the effect is the same as for pragma Suppress.  The meaning of the arguments is identical to that used in pragma Suppress.

One important application is to ensure that checks are on in cases where code depends on the checks for its correct functioning, so that the code will compile correctly even if the compiler switches are set to suppress checks.

Pragma Use_VADS_Size
====================

Syntax:

     pragma Use_VADS_Size;

This is a configuration pragma.  In a unit to which it applies, any use of the 'Size attribute is automatically interpreted as a use of the 'VADS_Size attribute.  Note that this may result in incorrect semantic processing of valid Ada 95 programs.  This is intended to aid in the handling of legacy code which depends on the interpretation of Size as implemented in the VADS compiler.  See description of the VADS_Size attribute for further details.

Pragma Validity_Checks
======================

Syntax:

     pragma Validity_Checks (string_LITERAL | ALL_CHECKS | On | Off);

This pragma is used in conjunction with compiler switches to control the built-in validity checking provided by GNAT.  The compiler switches, if set provide an initial setting for the switches, and this pragma may be used to modify these settings, or the settings may be provided entirely by the use of the pragma.  This pragma can be used anywhere that a pragma is legal, including use as a configuration pragma (including use in the gnat.adc file).

The form with a string literal specifies which validity options are to be activated.  The validity checks are first set to include only the default reference manual settings, and then a string of letters in the string specifies the exact set of options required.  The form of this string is exactly as described for the -gnatVx compiler switch (see the GNAT users guide for details).  For example the following two methods can be used to enable validity checking for mode in and in out subprogram parameters:

  * pragma Validity_Checks ("im");

  * gcc -c -gnatVim ...

The form ALL_CHECKS activates all standard checks (its use is equivalent to the use of the gnatva switch.

The forms with Off and On can be used to temporarily disable validity checks as shown in the following example:

     pragma Validity_Checks ("c"); -- validity checks for copies
     pragma Validity_Checks (Off); -- turn off validity checks
     A := B;                       -- B will not be validity checked
     pragma Validity_Checks (On);  -- turn validity checks back on
     A := C;                       -- C will be validity checked

Pragma Volatile
===============

Syntax:

     pragma Volatile (local_NAME);

This pragma is defined by the Ada 95 Reference Manual, and the GNAT implementation is fully conformant with this definition.  The reason it is mentioned in this section is that a pragma of the same name was supplied in some Ada 83 compilers, including DEC Ada 83.  The Ada 95 implementation of pragma Volatile is upwards compatible with the implementation in Dec Ada 83.

Pragma Warnings
===============

Syntax:

     pragma Warnings (On | Off [, LOCAL_NAME]);

Normally warnings are enabled, with the output being controlled by the command line switch.  Warnings (Off) turns off generation of warnings until a Warnings (On) is encountered or the end of the current unit.  If generation of warnings is turned off using this pragma, then no warning messages are output, regardless of the setting of the command line switches.

The form with a single argument is a configuration pragma.

If the LOCAL_NAME parameter is present, warnings are suppressed for the specified entity.  This suppression is effective from the point where it occurs till the end of the extended scope of the variable (similar to the scope of Suppress).

Pragma Weak_External
====================

Syntax:

     pragma Weak_External ([Entity =>] LOCAL_NAME);

This pragma specifies that the given entity should be marked as a weak external (one that does not have to be resolved) for the linker.  For further details, consult the GCC manual.

Implementation Defined Attributes
*********************************

Ada 95 defines (throughout the Ada 95 reference manual, summarized in annex K), a set of attributes that provide useful additional functionality in all areas of the language.  These language defined attributes are implemented in GNAT and work as described in the Ada 95 Reference Manual.

In addition, Ada 95 allows implementations to define additional attributes whose meaning is defined by the implementation.  GNAT provides a number of these implementation-dependent attributes which can be used to extend and enhance the functionality of the compiler.  This section of the GNAT reference manual describes these additional attributes.

Note that any program using these attributes may not be portable to other compilers (although GNAT implements this set of attributes on all platforms).  Therefore if portability to other compilers is an important consideration, you should minimize the use of these attributes.

Abort_Signal
============

Standard'Abort_Signal (Standard is the only allowed prefix) provides the entity for the special exception used to signal task abort or asynchronous transfer of control.  Normally this attribute should only be used in the tasking runtime (it is highly peculiar, and completely outside the normal semantics of Ada, for a user program to intercept the abort exception).

Address_Size
============

Standard'Address_Size (Standard is the only allowed prefix) is a static constant giving the number of bits in an Address. It is the same value as System.Address'Size, but has the advantage of being static, while a direct reference to System.Address'Size is non-static because Address is a private type.

Asm_Input
=========

The Asm_Input attribute denotes a function that takes two parameters.  The first is a string, the second is an expression of the type designated by the prefix.  The first (string) argument is required to be a static expression, and is the constraint for the parameter, (e.g. what kind of register is required).  The second argument is the value to be used as the input argument.  The possible values for the constant are the same as those used in the RTL, and are dependent on the configuration file used to built the GCC back end. Machine Code Insertions

Asm_Output
==========

The Asm_Output attribute denotes a function that takes two parameters.  The first is a string, the second is the name of a variable of the type designated by the attribute prefix.  The first (string) argument is required to be a static expression and designates the constraint for the parameter (e.g. what kind of register is required).  The second argument is the variable to be updated with the result.  The possible values for constraint are the same as those used in the RTL, and are dependent on the configuration file used to build the GCC back end.  If there are no output operands, then this argument may either be omitted, or explicitly given as No_Output_Operands. Machine Code Insertions

AST_Entry
=========

This attribute is implemented only in OpenVMS versions of GNAT.  Applied to the name of an entry, it yields a value of the predefined type AST_Handler (declared in the predefined package System, as extended by the use of pragma Extend_System (Aux_DEC)).  This value enables the given entry to be called when an AST occurs.  For further details, refer to the DEC Ada Language Reference Manual, section 9.12a.

Bit
===

OBJ'Bit, where OBJ is any object, yields the bit offset within the storage unit (byte) that contains the first bit of storage allocated for the object.  The value of this attribute is of the type Universal_Integer, and is always a non-negative number not exceeding the value of System.Storage_Unit.

For an object that is a variable or a constant allocated in a register, the value is zero.  (The use of this attribute does not force the allocation of a variable to memory).

For an object that is a formal parameter, this attribute applies to either the matching actual parameter or to a copy of the matching actual parameter.

For an access object the value is zero.  Note that OBJ.all'Bit is subject to an Access_Check for the designated object.  Similarly for a record component X.C'Bit is subject to a discriminant check and X(I).Bit and X(I1..I2)'Bit are subject to index checks.

This attribute is designed to be compatible with the DEC Ada 83 definition and implementation of the Bit attribute.

Bit_Position
============

R.C'Bit, where R is a record object and C is one of the fields of the record type, yields the bit offset within the record contains the first bit of storage allocated for the object.  The value of this attribute is of the type Universal_Integer.  The value depends only on the field C and is independent of the alignment of the containing record R.

Code_Address
============

The 'Address attribute may be applied to subprograms in Ada 95, but the intended effect from the Ada 95 reference manual seems to be to provide an address value which can be used to call the subprogram by means of an address clause as in the following example:

     procedure K is ...

     procedure L;
     for L'Address use K'Address;
     pragma Import (Ada, L);

A call to L is then expected to result in a call to K. In Ada 83, where there were no access-to-subprogram values, this was a common work around for getting the effect of an indirect call. GNAT implements the above use of Address and the technique illustrated by the example code works correctly.

However, for some purposes, it is useful to have the address of the start of the generated code for the subprogram.  On some architectures, this is not necessarily the same as the Address value described above. For example, the Address value may reference a subprogram descriptor rather than the subprogram itself.

The 'Code_Address attribute, which can only be applied to subprogram entities, always returns the address of the start of the generated code of the specified subprogram, which may or may not be the same value as is returned by the corresponding 'Address attribute.

Default_Bit_Order
=================

Standard'Default_Bit_Order (Standard is the only permissible prefix), provides the value System.Default_Bit_Order as a Pos value (0 for High_Order_First, 1 for Low_Order_First).  This is used to construct the definition of Default_Bit_Order in package System.

Elaborated
==========

The prefix of the 'Elaborated attribute must be a unit name.  The value is a Boolean which indicates whether or not the given unit has been elaborated.  This attribute is primarily intended for internal use by the generated code for dynamic elaboration checking, but it can also be used in user programs.  The value will always be True once elaboration of all units has been completed.  An exception is for units which need no elaboration, the value is always False for such units.

Elab_Body
=========

This attribute can only be applied to a program unit name.  It returns the entity for the corresponding elaboration procedure for elaborating the body of the referenced unit.  This is used in the main generated elaboration procedure by the binder and is not normally used in any other context.  However, there may be specialized situations in which it is useful to be able to call this elaboration procedure from Ada code, e.g. if it is necessary to do selective re-elaboration to fix some error.

Elab_Spec
=========

This attribute can only be applied to a program unit name.  It returns the entity for the corresponding elaboration procedure for elaborating the specification of the referenced unit.  This is used in the main generated elaboration procedure by the binder and is not normally used in any other context.  However, there may be specialized situations in which it is useful to be able to call this elaboration procedure from Ada code, e.g. if it is necessary to do selective re-elaboration to fix some error.

Emax
====

The Emax attribute is provided for compatibility with Ada 83.  See the Ada 83 reference manual for an exact description of the semantics of this attribute.

Enum_Rep
========

For every enumeration subtype S, S'Enum_Rep denotes a function with the following spec:

     function S'Enum_Rep (Arg : S'Base)
       return Universal_Integer;

It is also allowable to apply Enum_Rep directly to an object of an enumeration type or to a non-overloaded enumeration literal.  In this case S'Enum_Rep is equivalent to TYP'Enum_Rep(S) where TYP is the type of the enumeration literal or object.

The function returns the representation value for the given enumeration value.  This will be equal to value of the Pos attribute in the absence of an enumeration representation clause.  This is a static attribute (i.e. the result is static if the argument is static).

S'Enum_Rep can also be used with integer types and objects, in which case it simply returns the integer value.  The reason for this is to allow it to be used for (<>) discrete formal arguments in a generic unit that can be instantiated with either enumeration types or integer types.  Note that if Enum_Rep is used on a modular type whose upper bound exceeds the upper bound of the largest signed integer type, and the argument is a variable, so that the universal integer calculation is done at run-time, then the call to Enum_Rep may raise Constraint_Error.

Epsilon
=======

The Epsilon attribute is provided for compatibility with Ada 83.  See the Ada 83 reference manual for an exact description of the semantics of this attribute.

Fixed_Value
===========

For every fixed-point type S, S'Fixed_Value denotes a function with the following specification:

     function S'Fixed_Value (Arg : Universal_Integer)
       return S;

The value returned is the fixed-point value V such that

     V = Arg * S'Small

The effect is thus similar to first converting the argument to the integer type used to represent S, and then doing an unchecked conversion to the fixed-point type.  The difference is that there are full range checks, to ensure that the result is in range. This attribute is primarily intended for use in implementation of the input-output functions for fixed-point values.

Has_Access_Values
=================

The prefix of the Has_Access_Values attribute is a type.  The result is a Boolean value which is True if the is an access type, or is a composite type with a component (at any nesting depth) that is an access type, and is False otherwise. The intended use of this attribute is in conjunction with generic definitions.  If the attribute is applied to a generic private type, it indicates whether or not the corresponding actual type has access values.

Has_Discriminants
=================

The prefix of the Has_Discriminants attribute is a type.  The result is a Boolean value which is True if the type has discriminants, and False otherwise.  The intended use of this attribute is in conjunction with generic definitions.  If the attribute is applied to a generic private type, it indicates whether or not the corresponding actual type has discriminants.

Img
===

The Img attribute differs from Image in that it may be applied to objects as well as types, in which case it gives the Image for the subtype of the object.  This is convenient for debugging:

     Put_Line ("X = " & X'Img);

has the same meaning as the more verbose:

     Put_Line ("X = " & T'Image (X));

where T is the (sub)type of the object X.

Integer_Value
=============

For every integer type S, S'Integer_Value denotes a function with the following spec:

     function S'Integer_Value (Arg : Universal_Fixed)
       return S;

The value returned is the integer value V, such that

     Arg = V * T'Small

where T is the type of Arg. The effect is thus similar to first doing an unchecked conversion from the fixed-point type to its corresponding implementation type, and then converting the result to the target integer type.  The difference is that there are full range checks, to ensure that the result is in range. This attribute is primarily intended for use in implementation of the standard input-output functions for fixed-point values.

Large
=====

The Large attribute is provided for compatibility with Ada 83.  See the Ada 83 reference manual for an exact description of the semantics of this attribute.

Machine_Size
============

This attribute is identical to the Object_Size attribute.  It is provided for compatibility with the DEC Ada 83 attribute of this name.

Mantissa
========

The Mantissa attribute is provided for compatibility with Ada 83.  See the Ada 83 reference manual for an exact description of the semantics of this attribute.

Max_Interrupt_Priority
======================

Standard'Max_Interrupt_Priority (Standard is the only permissible prefix), provides the same value as System.Max_Interrupt_Priority.

Max_Priority
============

Standard'Max_Priority (Standard is the only permissible prefix) provides the same value as System.Max_Priority.

Maximum_Alignment
=================

Standard'Maximum_Alignment (Standard is the only permissible prefix) provides the maximum useful alignment value for the target.  This is a static value that can be used to specify the alignment for an object, guaranteeing that it is properly aligned in all cases.

Mechanism_Code
==============

FUNCTION'Mechanism_Code yields an integer code for the mechanism used for the result of function, and SUBPROGRAM'Mechanism_Code (N) yields the mechanism used for formal parameter number N (a static integer value with 1 meaning the first parameter) of SUBPROGRAM.  The code returned is:

1
     by copy (value)

2
     by reference

3
     by descriptor (default descriptor class)

4
     by descriptor (UBS: unaligned bit string)

5
     by descriptor (UBSB: aligned bit string with arbitrary bounds)

6
     by descriptor (UBA: unaligned bit array)

7
     by descriptor (S: string, also scalar access type parameter)

8
     by descriptor (SB: string with arbitrary bounds)

9
     by descriptor (A: contiguous array)

10
     by descriptor (NCA: non-contiguous array)

Values from 3 through 10 are only relevant to Digital OpenVMS implementations.

Null_Parameter
==============

A reference T'Null_Parameter denotes an imaginary object of type or subtype T allocated at machine address zero.  The attribute is allowed only as the default expression of a formal parameter, or as an actual expression of a subprogram call.  In either case, the subprogram must be imported.

The identity of the object is represented by the address zero in the argument list, independent of the passing mechanism (explicit or default).

This capability is needed to specify that a zero address should be passed for a record or other composite object passed by reference. There is no way of indicating this without the Null_Parameter attribute.

Object_Size
===========

The size of an object is not necessarily the same as the size of the type of an object.  This is because by default object sizes are increased to be a multiple of the alignment of the object.  For example, Natural'Size is 31, but by default objects of type Natural will have a size of 32 bits. Similarly, a record containing an integer and a character:

     type Rec is record
        I : Integer;
        C : Character;
     end record;

will have a size of 40 (that is Rec'Size will be 40.  The alignment will be 4, because of the integer field, and so the default size of record objects for this type will be 64 (8 bytes).

The TYPE'Object_Size attribute has been added to GNAT to allow the default object size of a type to be easily determined.  For example, Natural'Object_Size is 32, and Rec'Object_Size (for the record type in the above example) will be 64.  Note also that, unlike the situation with the Size attribute as defined in the Ada RM, the Object_Size attribute can be specified individually for different subtypes.  For example:

     type R is new Integer;
     subtype R1 is R range 1 .. 10;
     subtype R2 is R range 1 .. 10;
     for R2'Object_Size use 8;

In this example, R'Object_Size and R1'Object_Size are both 32 since the default object size for a subtype is the same as the object size for the parent subtype.  This means that objects of type R or R1 will by default be 32 bits (four bytes).  But objects of type R2 will be only 8 bits (one byte), since R2'Object_Size has been set to 8.

Passed_By_Reference
===================

TYPE'Passed_By_Reference for any subtype TYPE returns a value of type Boolean value that is True if the type is normally passed by reference and False if the type is normally passed by copy in calls.  For scalar types, the result is always False and is static.  For non-scalar types, the result is non-static.

Range_Length
============

TYPE'Range_Length for any discrete type TYPE yields the number of values represented by the subtype (zero for a null range).  The result is static for static subtypes.  Range_Length applied to the index subtype of a one dimensional array always gives the same result as Range applied to the array itself.

Safe_Emax
=========

The Safe_Emax attribute is provided for compatibility with Ada 83.  See the Ada 83 reference manual for an exact description of the semantics of this attribute.

Safe_Large
==========

The Safe_Large attribute is provided for compatibility with Ada 83.  See the Ada 83 reference manual for an exact description of the semantics of this attribute.

Small
=====

The Small attribute is defined in Ada 95 only for fixed-point types. GNAT also allows this attribute to be applied to floating-point types for compatibility with Ada 83.  See the Ada 83 reference manual for an exact description of the semantics of this attribute when applied to floating-point types.

Storage_Unit
============

Standard'Storage_Unit (Standard is the only permissible prefix) provides the same value as System.Storage_Unit.

Target_Name
===========

Standard'Target_Name (Standard is the only permissible prefix) provides a static string value that identifies the target for the current compilation. For GCC implementations, this is the standard gcc target name without the terminating slash (for example, GNAT 5.0 on windows yields "i586-pc-mingw32msv").

Tick
====

Standard'Tick (Standard is the only permissible prefix) provides the same value as System.Tick,

To_Address
==========

The System'To_Address (System is the only permissible prefix) denotes a function identical to System.Storage_Elements.To_Address except that it is a static attribute.  This means that if its argument is a static expression, then the result of the attribute is a static expression.  The result is that such an expression can be used in contexts (e.g. preelaborable packages) which require a static expression and where the function call could not be used (since the function call is always non-static, even if its argument is static).

Type_Class
==========

TYPE'Type_Class for any type or subtype TYPE yields the value of the type class for the full type of TYPE.  If TYPE is a generic formal type, the value is the value for the corresponding actual subtype.  The value of this attribute is of type System.Aux_DEC.Type_Class, which has the following definition:

       type Type_Class is
         (Type_Class_Enumeration,
          Type_Class_Integer,
          Type_Class_Fixed_Point,
          Type_Class_Floating_Point,
          Type_Class_Array,
          Type_Class_Record,
          Type_Class_Access,
          Type_Class_Task,
          Type_Class_Address);

Protected types yield the value Type_Class_Task, which thus applies to all concurrent types.  This attribute is designed to be compatible with the DEC Ada 83 attribute of the same name.

UET_Address
===========

The UET_Address attribute can only be used for a prefix which denotes a library package.  It yields the address of the unit exception table when zero cost exception handling is used.  This attribute is intended only for use within the GNAT implementation.  See the unit Ada.Exceptions in files a-except.ads and a-except.adb for details on how this attribute is used in the implementation.

Unconstrained_Array
===================

The Unconstrained_Array attribute can be used with a prefix that denotes any type or subtype. It is a static attribute that yields True if the prefix designates an unconstrained array, and False otherwise. In a generic instance, the result is still static, and yields the result of applying this test to the generic actual.

Universal_Literal_String
========================

The prefix of Universal_Literal_String must be a named number.  The static result is the string consisting of the characters of the number as defined in the original source.  This allows the user program to access the actual text of named numbers without intermediate conversions and without the need to enclose the strings in quotes (which would preclude their use as numbers).  This is used internally for the construction of values of the floating-point attributes from the file ttypef.ads, but may also be used by user programs.

Unrestricted_Access
===================

The Unrestricted_Access attribute is similar to Access except that all accessibility and aliased view checks are omitted.  This is a user-beware attribute.  It is similar to Address, for which it is a desirable replacement where the value desired is an access type.  In other words, its effect is identical to first applying the Address attribute and then doing an unchecked conversion to a desired access type.  In GNAT, but not necessarily in other implementations, the use of static chains for inner level subprograms means that Unrestricted_Access applied to a subprogram yields a value that can be called as long as the subprogram is in scope (normal Ada 95 accessibility rules restrict this usage).

It is possible to use Unrestricted_Access for any type, but care must be excercised if it is used to create pointers to unconstrained objects. In this case, the resulting pointer has the same scope as the context of the attribute, and may not be returned to some enclosing scope. For instance, a function cannot use Unrestricted_Access to create a unconstrained pointer and then return that value to the caller.

VADS_Size
=========

The 'VADS_Size attribute is intended to make it easier to port legacy code which relies on the semantics of 'Size as implemented by the VADS Ada 83 compiler.  GNAT makes a best effort at duplicating the same semantic interpretation.  In particular, 'VADS_Size applied to a predefined or other primitive type with no Size clause yields the Object_Size (for example, Natural'Size is 32 rather than 31 on typical machines).  In addition 'VADS_Size applied to an object gives the result that would be obtained by applying the attribute to the corresponding type.

Value_Size
==========

TYPE'Value_Size is the number of bits required to represent a value of the given subtype.  It is the same as TYPE'Size, but, unlike Size, may be set for non-first subtypes.

Wchar_T_Size
============

Standard'Wchar_T_Size (Standard is the only permissible prefix) provides the size in bits of the C wchar_t type primarily for constructing the definition of this type in package Interfaces.C.

Word_Size
=========

Standard'Word_Size (Standard is the only permissible prefix) provides the value System.Word_Size.

Implementation Advice
*********************

The main text of the Ada 95 Reference Manual describes the required behavior of all Ada 95 compilers, and the GNAT compiler conforms to these requirements.

In addition, there are sections throughout the Ada 95 reference manual headed by the phrase ``implementation advice''.  These sections are not normative, i.e. they do not specify requirements that all compilers must follow.  Rather they provide advice on generally desirable behavior.  You may wonder why they are not requirements.  The most typical answer is that they describe behavior that seems generally desirable, but cannot be provided on all systems, or which may be undesirable on some systems.

As far as practical, GNAT follows the implementation advice sections in the Ada 95 Reference Manual.  This chapter contains a table giving the reference manual section number, paragraph number and several keywords for each advice.  Each entry consists of the text of the advice followed by the GNAT interpretation of this advice.  Most often, this simply says ``followed'', which means that GNAT follows the advice.  However, in a number of cases, GNAT deliberately deviates from this advice, in which case the text describes what GNAT does and why.

1.1.3(20): Error Detection
==========================

If an implementation detects the use of an unsupported Specialized Needs Annex feature at run time, it should raise Program_Error if feasible.

Not relevant.  All specialized needs annex features are either supported, or diagnosed at compile time.

1.1.3(31): Child Units
======================

If an implementation wishes to provide implementation-defined extensions to the functionality of a language-defined library unit, it should normally do so by adding children to the library unit.

Followed.

1.1.5(12): Bounded Errors
=========================

If an implementation detects a bounded error or erroneous execution, it should raise Program_Error.

Followed in all cases in which the implementation detects a bounded error or erroneous execution.  Not all such situations are detected at runtime.

2.8(16): Pragmas
================

Normally, implementation-defined pragmas should have no semantic effect for error-free programs; that is, if the implementation-defined pragmas are removed from a working program, the program should still be legal, and should still have the same semantics.

The following implementation defined pragmas are exceptions to this rule:

Abort_Defer
     Affects semantics

Ada_83
     Affects legality

Assert
     Affects semantics

CPP_Class
     Affects semantics

CPP_Constructor
     Affects semantics

CPP_Virtual
     Affects semantics

CPP_Vtable
     Affects semantics

Debug
     Affects semantics

Interface_Name
     Affects semantics

Machine_Attribute
     Affects semantics

Unimplemented_Unit
     Affects legality

Unchecked_Union
     Affects semantics

In each of the above cases, it is essential to the purpose of the pragma that this advice not be followed.  For details see the separate section on implementation defined pragmas.

2.8(17-19): Pragmas
===================

Normally, an implementation should not define pragmas that can make an illegal program legal, except as follows:

A pragma used to complete a declaration, such as a pragma Import;

A pragma used to configure the environment by adding, removing, or replacing library_items.

See response to paragraph 16 of this same section.

3.5.2(5): Alternative Character Sets
====================================

If an implementation supports a mode with alternative interpretations for Character and Wide_Character, the set of graphic characters of Character should nevertheless remain a proper subset of the set of graphic characters of Wide_Character.  Any character set ``localizations'' should be reflected in the results of the subprograms defined in the language-defined package Characters.Handling (see A.3) available in such a mode.  In a mode with an alternative interpretation of Character, the implementation should also support a corresponding change in what is a legal identifier_letter.

Not all wide character modes follow this advice, in particular the JIS and IEC modes reflect standard usage in Japan, and in these encoding, the upper half of the Latin-1 set is not part of the wide-character subset, since the most significant bit is used for wide character encoding.  However, this only applies to the external forms.  Internally there is no such restriction.

3.5.4(28): Integer Types
========================

An implementation should support Long_Integer in addition to Integer if the target machine supports 32-bit (or longer) arithmetic.  No other named integer subtypes are recommended for package Standard.  Instead, appropriate named integer subtypes should be provided in the library package Interfaces (see B.2).

Long_Integer is supported.  Other standard integer types are supported so this advice is not fully followed.  These types are supported for convenient interface to C, and so that all hardware types of the machine are easily available.

3.5.4(29): Integer Types
========================

An implementation for a two's complement machine should support modular types with a binary modulus up to System.Max_Int*2+2.  An implementation should support a non-binary modules up to Integer'Last.

Followed.

3.5.5(8): Enumeration Values
============================

For the evaluation of a call on S'Pos for an enumeration subtype, if the value of the operand does not correspond to the internal code for any enumeration literal of its type (perhaps due to an un-initialized variable), then the implementation should raise Program_Error.  This is particularly important for enumeration types with noncontiguous internal codes specified by an enumeration_representation_clause.

Followed.

3.5.7(17): Float Types
======================

An implementation should support Long_Float in addition to Float if the target machine supports 11 or more digits of precision.  No other named floating point subtypes are recommended for package Standard.  Instead, appropriate named floating point subtypes should be provided in the library package Interfaces (see B.2).

Short_Float and Long_Long_Float are also provided.  The former provides improved compatibility with other implementations supporting this type.  The latter corresponds to the highest precision floating-point type supported by the hardware.  On most machines, this will be the same as Long_Float, but on some machines, it will correspond to the IEEE extended form.  The notable case is all ia32 (x86) implementations, where Long_Long_Float corresponds to the 80-bit extended precision format supported in hardware on this processor.  Note that the 128-bit format on SPARC is not supported, since this is a software rather than a hardware format.

3.6.2(11): Multidimensional Arrays
==================================

An implementation should normally represent multidimensional arrays in row-major order, consistent with the notation used for multidimensional array aggregates (see 4.3.3).  However, if a pragma Convention (Fortran, ...) applies to a multidimensional array type, then column-major order should be used instead (see B.5, ``Interfacing with Fortran'').

Followed.

9.6(30-31): Duration'Small
==========================

Whenever possible in an implementation, the value of Duration'Small should be no greater than 100 microseconds.

Followed.  (Duration'Small = 10**(-9)).

The time base for delay_relative_statements should be monotonic; it need not be the same time base as used for Calendar.Clock.

Followed.

10.2.1(12): Consistent Representation
=====================================

In an implementation, a type declared in a pre-elaborated package should have the same representation in every elaboration of a given version of the package, whether the elaborations occur in distinct executions of the same program, or in executions of distinct programs or partitions that include the given version.

Followed, except in the case of tagged types.  Tagged types involve implicit pointers to a local copy of a dispatch table, and these pointers have representations which thus depend on a particular elaboration of the package.  It is not easy to see how it would be possible to follow this advice without severely impacting efficiency of execution.

11.4.1(19): Exception Information
=================================

Exception_Message by default and Exception_Information should produce information useful for debugging.  Exception_Message should be short, about one line.  Exception_Information can be long.  Exception_Message should not include the Exception_Name.  Exception_Information should include both the Exception_Name and the Exception_Message.

Followed.  For each exception that doesn't have a specified Exception_Message, the compiler generates one containing the location of the raise statement.  This location has the form ``file:line'', where file is the short file name (without path information) and line is the line number in the file.  Note that in the case of the Zero Cost Exception mechanism, these messages become redundant with the Exception_Information that contains a full backtrace of the calling sequence, so they are disabled. To disable explicitly the generation of the source location message, use the Pragma Discard_Names.

11.5(28): Suppression of Checks
===============================

The implementation should minimize the code executed for checks that have been suppressed.

Followed.

13.1 (21-24): Representation Clauses
====================================

The recommended level of support for all representation items is qualified as follows:

An implementation need not support representation items containing non-static expressions, except that an implementation should support a representation item for a given entity if each non-static expression in the representation item is a name that statically denotes a constant declared before the entity.

Followed.  GNAT does not support non-static expressions in representation clauses unless they are constants declared before the entity.  For example:

     X : Some_Type;
     for X'Address use To_address (16#2000#);

will be rejected, since the To_Address expression is non-static.  Instead write:

     X_Address : constant Address : = To_Address (16#2000#);
     X         : Some_Type;
     for X'Address use X_Address;

An implementation need not support a specification for the Size for a given composite subtype, nor the size or storage place for an object (including a component) of a given composite subtype, unless the constraints on the subtype and its composite subcomponents (if any) are all static constraints.

Followed.  Size Clauses are not permitted on non-static components, as described above.

An aliased component, or a component whose type is by-reference, should always be allocated at an addressable location.

Followed.

13.2(6-8): Packed Types
=======================

If a type is packed, then the implementation should try to minimize storage allocated to objects of the type, possibly at the expense of speed of accessing components, subject to reasonable complexity in addressing calculations.

The recommended level of support pragma Pack is:

For a packed record type, the components should be packed as tightly as possible subject to the Sizes of the component subtypes, and subject to any record_representation_clause that applies to the type; the implementation may, but need not, reorder components or cross aligned word boundaries to improve the packing.  A component whose Size is greater than the word size may be allocated an integral number of words.

Followed.  Tight packing of arrays is supported for all component sizes up to 64-bits. If the array component size is 1 (that is to say, if the component is a boolean type or an enumeration type with two values) then values of the type are implicitly initialized to zero. This happens both for objects of the packed type, and for objects that have a subcomponent of the packed type.

An implementation should support Address clauses for imported subprograms.

Followed.

13.3(14-19): Address Clauses
============================

For an array X, X'Address should point at the first component of the array, and not at the array bounds.

Followed.

The recommended level of support for the Address attribute is:

X'Address should produce a useful result if X is an object that is aliased or of a by-reference type, or is an entity whose Address has been specified.

Followed.  A valid address will be produced even if none of those conditions have been met.  If necessary, the object is forced into memory to ensure the address is valid.

An implementation should support Address clauses for imported subprograms.

Followed.

Objects (including subcomponents) that are aliased or of a by-reference type should be allocated on storage element boundaries.

Followed.

If the Address of an object is specified, or it is imported or exported, then the implementation should not perform optimizations based on assumptions of no aliases.

Followed.

13.3(29-35): Alignment Clauses
==============================

The recommended level of support for the Alignment attribute for subtypes is:

An implementation should support specified Alignments that are factors and multiples of the number of storage elements per word, subject to the following:

Followed.

An implementation need not support specified Alignments for combinations of Sizes and Alignments that cannot be easily loaded and stored by available machine instructions.

Followed.

An implementation need not support specified Alignments that are greater than the maximum Alignment the implementation ever returns by default.

Followed.

The recommended level of support for the Alignment attribute for objects is:

Same as above, for subtypes, but in addition:

Followed.

For stand-alone library-level objects of statically constrained subtypes, the implementation should support all Alignments supported by the target linker.  For example, page alignment is likely to be supported for such objects, but not for subtypes.

Followed.

13.3(42-43): Size Clauses
=========================

The recommended level of support for the Size attribute of objects is:

A Size clause should be supported for an object if the specified Size is at least as large as its subtype's Size, and corresponds to a size in storage elements that is a multiple of the object's Alignment (if the Alignment is nonzero).

Followed.

13.3(50-56): Size Clauses
=========================

If the Size of a subtype is specified, and allows for efficient independent addressability (see 9.10) on the target architecture, then the Size of the following objects of the subtype should equal the Size of the subtype:

Aliased objects (including components).

Followed.

Size clause on a composite subtype should not affect the internal layout of components.

Followed.

The recommended level of support for the Size attribute of subtypes is:

The Size (if not specified) of a static discrete or fixed point subtype should be the number of bits needed to represent each value belonging to the subtype using an unbiased representation, leaving space for a sign bit only if the subtype contains negative values.  If such a subtype is a first subtype, then an implementation should support a specified Size for it that reflects this representation.

Followed.

For a subtype implemented with levels of indirection, the Size should include the size of the pointers, but not the size of what they point at.

Followed.

13.3(71-73): Component Size Clauses
===================================

The recommended level of support for the Component_Size attribute is:

An implementation need not support specified Component_Sizes that are less than the Size of the component subtype.

Followed.

An implementation should support specified Component_Sizes that are factors and multiples of the word size.  For such Component_Sizes, the array should contain no gaps between components.  For other Component_Sizes (if supported), the array should contain no gaps between components when packing is also specified; the implementation should forbid this combination in cases where it cannot support a no-gaps representation.

Followed.

13.4(9-10): Enumeration Representation Clauses
==============================================

The recommended level of support for enumeration representation clauses is:

An implementation need not support enumeration representation clauses for boolean types, but should at minimum support the internal codes in the range System.Min_Int.System.Max_Int.

Followed.

13.5.1(17-22): Record Representation Clauses
============================================

The recommended level of support for

record_representation_clauses is:

An implementation should support storage places that can be extracted with a load, mask, shift sequence of machine code, and set with a load, shift, mask, store sequence, given the available machine instructions and run-time model.

Followed.

A storage place should be supported if its size is equal to the Size of the component subtype, and it starts and ends on a boundary that obeys the Alignment of the component subtype.

Followed.

If the default bit ordering applies to the declaration of a given type, then for a component whose subtype's Size is less than the word size, any storage place that does not cross an aligned word boundary should be supported.

Followed.

An implementation may reserve a storage place for the tag field of a tagged type, and disallow other components from overlapping that place.

Followed.  The storage place for the tag field is the beginning of the tagged record, and its size is Address'Size.  GNAT will reject an explicit component clause for the tag field.

An implementation need not support a component_clause for a component of an extension part if the storage place is not after the storage places of all components of the parent type, whether or not those storage places had been specified.

Followed.  The above advice on record representation clauses is followed, and all mentioned features are implemented.

13.5.2(5): Storage Place Attributes
===================================

If a component is represented using some form of pointer (such as an offset) to the actual data of the component, and this data is contiguous with the rest of the object, then the storage place attributes should reflect the place of the actual data, not the pointer.  If a component is allocated discontinuously from the rest of the object, then a warning should be generated upon reference to one of its storage place attributes.

Followed.  There are no such components in GNAT.

13.5.3(7-8): Bit Ordering
=========================

The recommended level of support for the non-default bit ordering is:

If Word_Size = Storage_Unit, then the implementation should support the non-default bit ordering in addition to the default bit ordering.

Followed.  Word size does not equal storage size in this implementation. Thus non-default bit ordering is not supported.

13.7(37): Address as Private
============================

Address should be of a private type.

Followed.

13.7.1(16): Address Operations
==============================

Operations in System and its children should reflect the target environment semantics as closely as is reasonable.  For example, on most machines, it makes sense for address arithmetic to ``wrap around''. Operations that do not make sense should raise Program_Error.

Followed.  Address arithmetic is modular arithmetic that wraps around.  No operation raises Program_Error, since all operations make sense.

13.9(14-17): Unchecked Conversion
=================================

The Size of an array object should not include its bounds; hence, the bounds should not be part of the converted data.

Followed.

The implementation should not generate unnecessary run-time checks to ensure that the representation of S is a representation of the target type.  It should take advantage of the permission to return by reference when possible.  Restrictions on unchecked conversions should be avoided unless required by the target environment.

Followed.  There are no restrictions on unchecked conversion.  A warning is generated if the source and target types do not have the same size since the semantics in this case may be target dependent.

The recommended level of support for unchecked conversions is:

Unchecked conversions should be supported and should be reversible in the cases where this clause defines the result.  To enable meaningful use of unchecked conversion, a contiguous representation should be used for elementary subtypes, for statically constrained array subtypes whose component subtype is one of the subtypes described in this paragraph, and for record subtypes without discriminants whose component subtypes are described in this paragraph.

Followed.

13.11(23-25): Implicit Heap Usage
=================================

An implementation should document any cases in which it dynamically allocates heap storage for a purpose other than the evaluation of an allocator.

Followed, the only other points at which heap storage is dynamically allocated are as follows:

  * At initial elaboration time, to allocate dynamically sized global objects.

  * To allocate space for a task when a task is created.

  * To extend the secondary stack dynamically when needed.  The secondary stack is used for returning variable length results.

A default (implementation-provided) storage pool for an access-to-constant type should not have overhead to support deallocation of individual objects.

Followed.

A storage pool for an anonymous access type should be created at the point of an allocator for the type, and be reclaimed when the designated object becomes inaccessible.

Followed.

13.11.2(17): Unchecked De-allocation
====================================

For a standard storage pool, Free should actually reclaim the storage.

Followed.

13.13.2(17): Stream Oriented Attributes
=======================================

If a stream element is the same size as a storage element, then the normal in-memory representation should be used by Read and Write for scalar objects.  Otherwise, Read and Write should use the smallest number of stream elements needed to represent all values in the base range of the scalar type.

Followed.  By default, GNAT uses the interpretation suggested by AI-195, which specifies using the size of the first subtype. However, such an implementation is based on direct binary representations and is therefore target- and endianness-dependent. To address this issue, GNAT also supplies an alternate implementation of the stream attributes Read and Write, which uses the target-independent XDR standard representation for scalar types. The XDR implementation is provided as an alternative body of the System.Stream_Attributes package, in the file s-strxdr.adb in the GNAT library. There is no s-strxdr.ads file. In order to install the XDR implementation, do the following:

 1. Replace the default implementation of the System.Stream_Attributes package with the XDR implementation. For example on a Unix platform issue the commands:

          $ mv s-stratt.adb s-strold.adb
          $ mv s-strxdr.adb s-stratt.adb

 2. Rebuild the GNAT run-time library as documented in the GNAT User's Guide

A.1(52): Names of Predefined Numeric Types
==========================================

If an implementation provides additional named predefined integer types, then the names should end with Integer as in Long_Integer.  If an implementation provides additional named predefined floating point types, then the names should end with Float as in Long_Float.

Followed.

A.3.2(49): Ada.Characters.Handling
==================================

If an implementation provides a localized definition of Character or Wide_Character, then the effects of the subprograms in Characters.Handling should reflect the localizations.  See also 3.5.2.

Followed.  GNAT provides no such localized definitions.

A.4.4(106): Bounded-Length String Handling
==========================================

Bounded string objects should not be implemented by implicit pointers and dynamic allocation.

Followed.  No implicit pointers or dynamic allocation are used.

A.5.2(46-47): Random Number Generation
======================================

Any storage associated with an object of type Generator should be reclaimed on exit from the scope of the object.

Followed.

If the generator period is sufficiently long in relation to the number of distinct initiator values, then each possible value of Initiator passed to Reset should initiate a sequence of random numbers that does not, in a practical sense, overlap the sequence initiated by any other value.  If this is not possible, then the mapping between initiator values and generator states should be a rapidly varying function of the initiator value.

Followed.  The generator period is sufficiently long for the first condition here to hold true.

A.10.7(23): Get_Immediate
=========================

The Get_Immediate procedures should be implemented with unbuffered input.  For a device such as a keyboard, input should be available if a key has already been typed, whereas for a disk file, input should always be available except at end of file.  For a file associated with a keyboard-like device, any line-editing features of the underlying operating system should be disabled during the execution of Get_Immediate.

Followed on all targets except VxWorks. For VxWorks, there is no way to provide this functionality that does not result in the input buffer being flushed before the Get_Immediate call. A special unit Interfaces.Vxworks.IO is provided that contains routines to enable this functionality.

B.1(39-41): Pragma Export
=========================

If an implementation supports pragma Export to a given language, then it should also allow the main subprogram to be written in that language.  It should support some mechanism for invoking the elaboration of the Ada library units included in the system, and for invoking the finalization of the environment task.  On typical systems, the recommended mechanism is to provide two subprograms whose link names are adainit and adafinal.  adainit should contain the elaboration code for library units.  adafinal should contain the finalization code.  These subprograms should have no effect the second and subsequent time they are called.

Followed.

Automatic elaboration of pre-elaborated packages should be provided when pragma Export is supported.

Followed when the main program is in Ada.  If the main program is in a foreign language, then adainit must be called to elaborate pre-elaborated packages.

For each supported convention L other than Intrinsic, an implementation should support Import and Export pragmas for objects of L-compatible types and for subprograms, and pragma Convention for L-eligible types and for subprograms, presuming the other language has corresponding features.  Pragma Convention need not be supported for scalar types.

Followed.

B.2(12-13): Package Interfaces
==============================

For each implementation-defined convention identifier, there should be a child package of package Interfaces with the corresponding name.  This package should contain any declarations that would be useful for interfacing to the language (implementation) represented by the convention.  Any declarations useful for interfacing to any language on the given hardware architecture should be provided directly in Interfaces.

Followed.  An additional package not defined in the Ada 95 Reference Manual is Interfaces.CPP, used for interfacing to C++.

An implementation supporting an interface to C, COBOL, or Fortran should provide the corresponding package or packages described in the following clauses.

Followed.  GNAT provides all the packages described in this section.

B.3(63-71): Interfacing with C
==============================

An implementation should support the following interface correspondences between Ada and C.

Followed.

An Ada procedure corresponds to a void-returning C function.

Followed.

An Ada function corresponds to a non-void C function.

Followed.

An Ada in scalar parameter is passed as a scalar argument to a C function.

Followed.

An Ada in parameter of an access-to-object type with designated type T is passed as a T* argument to a C function, where T is the C type corresponding to the Ada type T.

Followed.

An Ada access T parameter, or an Ada out or in out parameter of an elementary type T, is passed as a T* argument to a C function, where T is the C type corresponding to the Ada type T.  In the case of an elementary out or in out parameter, a pointer to a temporary copy is used to preserve by-copy semantics.

Followed.

An Ada parameter of a record type T, of any mode, is passed as a T* argument to a C function, where T is the C structure corresponding to the Ada type T.

Followed.  This convention may be overridden by the use of the C_Pass_By_Copy pragma, or Convention, or by explicitly specifying the mechanism for a given call using an extended import or export pragma.

An Ada parameter of an array type with component type T, of any mode, is passed as a T* argument to a C function, where T is the C type corresponding to the Ada type T.

Followed.

An Ada parameter of an access-to-subprogram type is passed as a pointer to a C function whose prototype corresponds to the designated subprogram's specification.

Followed.

B.4(95-98): Interfacing with COBOL
==================================

An Ada implementation should support the following interface correspondences between Ada and COBOL.

Followed.

An Ada access T parameter is passed as a BY REFERENCE data item of the COBOL type corresponding to T.

Followed.

An Ada in scalar parameter is passed as a BY CONTENT data item of the corresponding COBOL type.

Followed.

Any other Ada parameter is passed as a BY REFERENCE data item of the COBOL type corresponding to the Ada parameter type; for scalars, a local copy is used if necessary to ensure by-copy semantics.

Followed.

B.5(22-26): Interfacing with Fortran
====================================

An Ada implementation should support the following interface correspondences between Ada and Fortran:

Followed.

An Ada procedure corresponds to a Fortran subroutine.

Followed.

An Ada function corresponds to a Fortran function.

Followed.

An Ada parameter of an elementary, array, or record type T is passed as a T argument to a Fortran procedure, where T is the Fortran type corresponding to the Ada type T, and where the INTENT attribute of the corresponding dummy argument matches the Ada formal parameter mode; the Fortran implementation's parameter passing conventions are used.  For elementary types, a local copy is used if necessary to ensure by-copy semantics.

Followed.

An Ada parameter of an access-to-subprogram type is passed as a reference to a Fortran procedure whose interface corresponds to the designated subprogram's specification.

Followed.

C.1(3-5): Access to Machine Operations
======================================

The machine code or intrinsic support should allow access to all operations normally available to assembly language programmers for the target environment, including privileged instructions, if any.

Followed.

The interfacing pragmas (see Annex B) should support interface to assembler; the default assembler should be associated with the convention identifier Assembler.

Followed.

If an entity is exported to assembly language, then the implementation should allocate it at an addressable location, and should ensure that it is retained by the linking process, even if not otherwise referenced from the Ada code.  The implementation should assume that any call to a machine code or assembler subprogram is allowed to read or update every object that is specified as exported.

Followed.

C.1(10-16): Access to Machine Operations
========================================

The implementation should ensure that little or no overhead is associated with calling intrinsic and machine-code subprograms.

Followed for both intrinsics and machine-code subprograms.

It is recommended that intrinsic subprograms be provided for convenient access to any machine operations that provide special capabilities or efficiency and that are not otherwise available through the language constructs.

Followed.  A full set of machine operation intrinsic subprograms is provided.

Atomic read-modify-write operations---e.g., test and set, compare and swap, decrement and test, enqueue/dequeue.

Followed on any target supporting such operations.

Standard numeric functions---e.g., sin, log.

Followed on any target supporting such operations.

String manipulation operations---e.g., translate and test.

Followed on any target supporting such operations.

Vector operations---e.g., compare vector against thresholds.

Followed on any target supporting such operations.

Direct operations on I/O ports.

Followed on any target supporting such operations.

C.3(28): Interrupt Support
==========================

If the Ceiling_Locking policy is not in effect, the implementation should provide means for the application to specify which interrupts are to be blocked during protected actions, if the underlying system allows for a finer-grain control of interrupt blocking.

Followed.  The underlying system does not allow for finer-grain control of interrupt blocking.

C.3.1(20-21): Protected Procedure Handlers
==========================================

Whenever possible, the implementation should allow interrupt handlers to be called directly by the hardware.

Followed on any target where the underlying operating system permits such direct calls.

Whenever practical, violations of any implementation-defined restrictions should be detected before run time.

Followed.  Compile time warnings are given when possible.

C.3.2(25): Package Interrupts
=============================

If implementation-defined forms of interrupt handler procedures are supported, such as protected procedures with parameters, then for each such form of a handler, a type analogous to Parameterless_Handler should be specified in a child package of Interrupts, with the same operations as in the predefined package Interrupts.

Followed.

C.4(14): Pre-elaboration Requirements
=====================================

It is recommended that pre-elaborated packages be implemented in such a way that there should be little or no code executed at run time for the elaboration of entities not already covered by the Implementation Requirements.

Followed.  Executable code is generated in some cases, e.g. loops to initialize large arrays.

C.5(8): Pragma Discard_Names
============================

If the pragma applies to an entity, then the implementation should reduce the amount of storage used for storing names associated with that entity.

Followed.

C.7.2(30): The Package Task_Attributes
======================================

Some implementations are targeted to domains in which memory use at run time must be completely deterministic.  For such implementations, it is recommended that the storage for task attributes will be pre-allocated statically and not from the heap.  This can be accomplished by either placing restrictions on the number and the size of the task's attributes, or by using the pre-allocated storage for the first N attribute objects, and the heap for the others.  In the latter case, N should be documented.

Not followed.  This implementation is not targeted to such a domain.

D.3(17): Locking Policies
=========================

The implementation should use names that end with _Locking for locking policies defined by the implementation.

Followed.  A single implementation-defined locking policy is defined, whose name (Inheritance_Locking) follows this suggestion.

D.4(16): Entry Queuing Policies
===============================

Names that end with _Queuing should be used for all implementation-defined queuing policies.

Followed.  No such implementation-defined queuing policies exist.

D.6(9-10): Preemptive Abort
===========================

Even though the abort_statement is included in the list of potentially blocking operations (see 9.5.1), it is recommended that this statement be implemented in a way that never requires the task executing the abort_statement to block.

Followed.

On a multi-processor, the delay associated with aborting a task on another processor should be bounded; the implementation should use periodic polling, if necessary, to achieve this.

Followed.

D.7(21): Tasking Restrictions
=============================

When feasible, the implementation should take advantage of the specified restrictions to produce a more efficient implementation.

GNAT currently takes advantage of these restrictions by providing an optimized run time when the Ravenscar profile and the GNAT restricted run time set of restrictions are specified.  See pragma Profile (Ravenscar) and pragma Profile (Restricted) for more details.

D.8(47-49): Monotonic Time
==========================

When appropriate, implementations should provide configuration mechanisms to change the value of Tick.

Such configuration mechanisms are not appropriate to this implementation and are thus not supported.

It is recommended that Calendar.Clock and Real_Time.Clock be implemented as transformations of the same time base.

Followed.

It is recommended that the best time base which exists in the underlying system be available to the application through Clock.  Best may mean highest accuracy or largest range.

Followed.

E.5(28-29): Partition Communication Subsystem
=============================================

Whenever possible, the PCS on the called partition should allow for multiple tasks to call the RPC-receiver with different messages and should allow them to block until the corresponding subprogram body returns.

Followed by GLADE, a separately supplied PCS that can be used with GNAT.

The Write operation on a stream of type Params_Stream_Type should raise Storage_Error if it runs out of space trying to write the Item into the stream.

Followed by GLADE, a separately supplied PCS that can be used with GNAT.

F(7): COBOL Support
===================

If COBOL (respectively, C) is widely supported in the target environment, implementations supporting the Information Systems Annex should provide the child package Interfaces.COBOL (respectively, Interfaces.C) specified in Annex B and should support a convention_identifier of COBOL (respectively, C) in the interfacing pragmas (see Annex B), thus allowing Ada programs to interface with programs written in that language.

Followed.

F.1(2): Decimal Radix Support
=============================

Packed decimal should be used as the internal representation for objects of subtype S when S'Machine_Radix = 10.

Not followed.  GNAT ignores S'Machine_Radix and always uses binary representations.

G: Numerics
===========

If Fortran (respectively, C) is widely supported in the target environment, implementations supporting the Numerics Annex should provide the child package Interfaces.Fortran (respectively, Interfaces.C) specified in Annex B and should support a convention_identifier of Fortran (respectively, C) in the interfacing pragmas (see Annex B), thus allowing Ada programs to interface with programs written in that language.

Followed.

G.1.1(56-58): Complex Types
===========================

Because the usual mathematical meaning of multiplication of a complex operand and a real operand is that of the scaling of both components of the former by the latter, an implementation should not perform this operation by first promoting the real operand to complex type and then performing a full complex multiplication.  In systems that, in the future, support an Ada binding to IEC 559:1989, the latter technique will not generate the required result when one of the components of the complex operand is infinite.  (Explicit multiplication of the infinite component by the zero component obtained during promotion yields a NaN that propagates into the final result.) Analogous advice applies in the case of multiplication of a complex operand and a pure-imaginary operand, and in the case of division of a complex operand by a real or pure-imaginary operand.

Not followed.

Similarly, because the usual mathematical meaning of addition of a complex operand and a real operand is that the imaginary operand remains unchanged, an implementation should not perform this operation by first promoting the real operand to complex type and then performing a full complex addition.  In implementations in which the Signed_Zeros attribute of the component type is True (and which therefore conform to IEC 559:1989 in regard to the handling of the sign of zero in predefined arithmetic operations), the latter technique will not generate the required result when the imaginary component of the complex operand is a negatively signed zero.  (Explicit addition of the negative zero to the zero obtained during promotion yields a positive zero.) Analogous advice applies in the case of addition of a complex operand and a pure-imaginary operand, and in the case of subtraction of a complex operand and a real or pure-imaginary operand.

Not followed.

Implementations in which Real'Signed_Zeros is True should attempt to provide a rational treatment of the signs of zero results and result components.  As one example, the result of the Argument function should have the sign of the imaginary component of the parameter X when the point represented by that parameter lies on the positive real axis; as another, the sign of the imaginary component of the Compose_From_Polar function should be the same as (respectively, the opposite of) that of the Argument parameter when that parameter has a value of zero and the Modulus parameter has a nonnegative (respectively, negative) value.

Followed.

G.1.2(49): Complex Elementary Functions
=======================================

Implementations in which Complex_Types.Real'Signed_Zeros is True should attempt to provide a rational treatment of the signs of zero results and result components.  For example, many of the complex elementary functions have components that are odd functions of one of the parameter components; in these cases, the result component should have the sign of the parameter component at the origin.  Other complex elementary functions have zero components whose sign is opposite that of a parameter component at the origin, or is always positive or always negative.

Followed.

G.2.4(19): Accuracy Requirements
================================

The versions of the forward trigonometric functions without a Cycle parameter should not be implemented by calling the corresponding version with a Cycle parameter of 2.0*Numerics.Pi, since this will not provide the required accuracy in some portions of the domain.  For the same reason, the version of Log without a Base parameter should not be implemented by calling the corresponding version with a Base parameter of Numerics.e.

Followed.

G.2.6(15): Complex Arithmetic Accuracy
======================================

The version of the Compose_From_Polar function without a Cycle parameter should not be implemented by calling the corresponding version with a Cycle parameter of 2.0*Numerics.Pi, since this will not provide the required accuracy in some portions of the domain.

Followed.

Implementation Defined Characteristics
**************************************

In addition to the implementation dependent pragmas and attributes, and the implementation advice, there are a number of other features of Ada 95 that are potentially implementation dependent.  These are mentioned throughout the Ada 95 Reference Manual, and are summarized in annex M.

A requirement for conforming Ada compilers is that they provide documentation describing how the implementation deals with each of these issues.  In this chapter, you will find each point in annex M listed followed by a description in italic font of how GNAT handles the implementation dependence.

You can use this chapter as a guide to minimizing implementation dependent features in your programs if portability to other compilers and other operating systems is an important consideration.  The numbers in each section below correspond to the paragraph number in the Ada 95 Reference Manual.

2.  Whether or not each recommendation given in Implementation Advice is followed.  See 1.1.2(37).

See Implementation Advice.

3.  Capacity limitations of the implementation.  See 1.1.3(3).

The complexity of programs that can be processed is limited only by the total amount of available virtual memory, and disk space for the generated object files.

4.  Variations from the standard that are impractical to avoid given the implementation's execution environment.  See 1.1.3(6).

There are no variations from the standard.

5.  Which code_statements cause external interactions.  See 1.1.3(10).

Any code_statement can potentially cause external interactions.

6.  The coded representation for the text of an Ada program.  See 2.1(4).

See separate section on source representation.

7.  The control functions allowed in comments.  See 2.1(14).

See separate section on source representation.

8.  The representation for an end of line.  See 2.2(2).

See separate section on source representation.

9.  Maximum supported line length and lexical element length.  See 2.2(15).

The maximum line length is 255 characters an the maximum length of a lexical element is also 255 characters.

10.  Implementation defined pragmas.  See 2.8(14).

See Implementation Defined Pragmas.

11.  Effect of pragma Optimize.  See 2.8(27).

Pragma Optimize, if given with a Time or Space parameter, checks that the optimization flag is set, and aborts if it is not.

12.  The sequence of characters of the value returned by S'Image when some of the graphic characters of S'Wide_Image are not defined in Character.  See 3.5(37).

The sequence of characters is as defined by the wide character encoding method used for the source.  See section on source representation for further details.

13.  The predefined integer types declared in Standard.  See 3.5.4(25).

Short_Short_Integer
     8 bit signed

Short_Integer
     (Short) 16 bit signed

Integer
     32 bit signed

Long_Integer
     64 bit signed (Alpha OpenVMS only) 32 bit signed (all other targets)

Long_Long_Integer
     64 bit signed

14.  Any nonstandard integer types and the operators defined for them.  See 3.5.4(26).

There are no nonstandard integer types.

15.  Any nonstandard real types and the operators defined for them.  See 3.5.6(8).

There are no nonstandard real types.

16.  What combinations of requested decimal precision and range are supported for floating point types.  See 3.5.7(7).

The precision and range is as defined by the IEEE standard.

17.  The predefined floating point types declared in Standard.  See 3.5.7(16).

Short_Float
     32 bit IEEE short

Float
     (Short) 32 bit IEEE short

Long_Float
     64 bit IEEE long

Long_Long_Float
     64 bit IEEE long (80 bit IEEE long on x86 processors)

18.  The small of an ordinary fixed point type.  See 3.5.9(8).

Fine_Delta is 2**(-63)

19.  What combinations of small, range, and digits are supported for fixed point types.  See 3.5.9(10).

Any combinations are permitted that do not result in a small less than Fine_Delta and do not result in a mantissa larger than 63 bits. If the mantissa is larger than 53 bits on machines where Long_Long_Float is 64 bits (true of all architectures except ia32), then the output from Text_IO is accurate to only 53 bits, rather than the full mantissa.  This is because floating-point conversions are used to convert fixed point.

20.  The result of Tags.Expanded_Name for types declared within an unnamed block_statement.  See 3.9(10).

Block numbers of the form BNNN, where NNN is a decimal integer are allocated.

21.  Implementation-defined attributes.  See 4.1.4(12).

See Implementation Defined Attributes.

22.  Any implementation-defined time types.  See 9.6(6).

There are no implementation-defined time types.

23.  The time base associated with relative delays.

See 9.6(20).  The time base used is that provided by the C library function gettimeofday.

24.  The time base of the type Calendar.Time.  See 9.6(23).

The time base used is that provided by the C library function gettimeofday.

25.  The time zone used for package Calendar operations.  See 9.6(24).

The time zone used by package Calendar is the current system time zone setting for local time, as accessed by the C library function localtime.

26.  Any limit on delay_until_statements of select_statements.  See 9.6(29).

There are no such limits.

27.  Whether or not two non overlapping parts of a composite object are independently addressable, in the case where packing, record layout, or Component_Size is specified for the object.  See 9.10(1).

Separate components are independently addressable if they do not share overlapping storage units.

28.  The representation for a compilation.  See 10.1(2).

A compilation is represented by a sequence of files presented to the compiler in a single invocation of the gcc command.

29.  Any restrictions on compilations that contain multiple compilation_units.  See 10.1(4).

No single file can contain more than one compilation unit, but any sequence of files can be presented to the compiler as a single compilation.

30.  The mechanisms for creating an environment and for adding and replacing compilation units.  See 10.1.4(3).

See separate section on compilation model.

31.  The manner of explicitly assigning library units to a partition.  See 10.2(2).

If a unit contains an Ada main program, then the Ada units for the partition are determined by recursive application of the rules in the Ada Reference Manual section 10.2(2-6).  In other words, the Ada units will be those that are needed by the main program, and then this definition of need is applied recursively to those units, and the partition contains the transitive closure determined by this relationship.  In short, all the necessary units are included, with no need to explicitly specify the list.  If additional units are required, e.g. by foreign language units, then all units must be mentioned in the context clause of one of the needed Ada units.

If the partition contains no main program, or if the main program is in a language other than Ada, then GNAT provides the binder options -z and -n respectively, and in this case a list of units can be explicitly supplied to the binder for inclusion in the partition (all units needed by these units will also be included automatically).  For full details on the use of these options, refer to the GNAT User's Guide sections on Binding and Linking.

32.  The implementation-defined means, if any, of specifying which compilation units are needed by a given compilation unit.  See 10.2(2).

The units needed by a given compilation unit are as defined in the Ada Reference Manual section 10.2(2-6).  There are no implementation-defined pragmas or other implementation-defined means for specifying needed units.

33.  The manner of designating the main subprogram of a partition.  See 10.2(7).

The main program is designated by providing the name of the corresponding ALI file as the input parameter to the binder.

34.  The order of elaboration of library_items.  See 10.2(18).

The first constraint on ordering is that it meets the requirements of chapter 10 of the Ada 95 Reference Manual.  This still leaves some implementation dependent choices, which are resolved by first elaborating bodies as early as possible (i.e. in preference to specs where there is a choice), and second by evaluating the immediate with clauses of a unit to determine the probably best choice, and third by elaborating in alphabetical order of unit names where a choice still remains.

35.  Parameter passing and function return for the main subprogram.  See 10.2(21).

The main program has no parameters.  It may be a procedure, or a function returning an integer type.  In the latter case, the returned integer value is the return code of the program (overriding any value that may have been set by a call to Ada.Command_Line.Set_Exit_Status).

36.  The mechanisms for building and running partitions.  See 10.2(24).

GNAT itself supports programs with only a single partition.  The GNATDIST tool provided with the GLADE package (which also includes an implementation of the PCS) provides a completely flexible method for building and running programs consisting of multiple partitions.  See the separate GLADE manual for details.

37.  The details of program execution, including program termination.  See 10.2(25).

See separate section on compilation model.

38.  The semantics of any non-active partitions supported by the implementation.  See 10.2(28).

Passive partitions are supported on targets where shared memory is provided by the operating system.  See the GLADE reference manual for further details.

39.  The information returned by Exception_Message.  See 11.4.1(10).

Exception message returns the null string unless a specific message has been passed by the program.

40.  The result of Exceptions.Exception_Name for types declared within an unnamed block_statement.  See 11.4.1(12).

Blocks have implementation defined names of the form BNNN where NNN is an integer.

41.  The information returned by Exception_Information.  See 11.4.1(13).

Exception_Information returns a string in the following format:

     Exception_Name: nnnnn
     Message: mmmmm
     PID: ppp
     Call stack traceback locations:
     0xhhhh 0xhhhh 0xhhhh ... 0xhhh

where

  * nnnn is the fully qualified name of the exception in all upper case letters. This line is always present.

  * mmmm is the message (this line present only if message is non-null)

  * ppp is the Process Id value as a decimal integer (this line is present only if the Process Id is non-zero). Currently we are not making use of this field.

  * The Call stack traceback locations line and the following values are present only if at least one traceback location was recorded. The values are given in C style format, with lower case letters for a-f, and only as many digits present as are necessary.

The line terminator sequence at the end of each line, including the last line is a single LF character (16#0A#).

42.  Implementation-defined check names.  See 11.5(27).

No implementation-defined check names are supported.

43.  The interpretation of each aspect of representation.  See 13.1(20).

See separate section on data representations.

44.  Any restrictions placed upon representation items.  See 13.1(20).

See separate section on data representations.

45.  The meaning of Size for indefinite subtypes.  See 13.3(48).

Size for an indefinite subtype is the maximum possible size, except that for the case of a subprogram parameter, the size of the parameter object is the actual size.

46.  The default external representation for a type tag.  See 13.3(75).

The default external representation for a type tag is the fully expanded name of the type in upper case letters.

47.  What determines whether a compilation unit is the same in two different partitions.  See 13.3(76).

A compilation unit is the same in two different partitions if and only if it derives from the same source file.

48.  Implementation-defined components.  See 13.5.1(15).

The only implementation defined component is the tag for a tagged type, which contains a pointer to the dispatching table.

49.  If Word_Size = Storage_Unit, the default bit ordering.  See 13.5.3(5).

Word_Size (32) is not the same as Storage_Unit (8) for this implementation, so no non-default bit ordering is supported.  The default bit ordering corresponds to the natural endianness of the target architecture.

50.  The contents of the visible part of package System and its language-defined children.  See 13.7(2).

See the definition of these packages in files system.ads and s-stoele.ads.

51.  The contents of the visible part of package System.Machine_Code, and the meaning of code_statements.  See 13.8(7).

See the definition and documentation in file s-maccod.ads.

52.  The effect of unchecked conversion.  See 13.9(11).

Unchecked conversion between types of the same size and results in an uninterpreted transmission of the bits from one type to the other.  If the types are of unequal sizes, then in the case of discrete types, a shorter source is first zero or sign extended as necessary, and a shorter target is simply truncated on the left. For all non-discrete types, the source is first copied if necessary to ensure that the alignment requirements of the target are met, then a pointer is constructed to the source value, and the result is obtained by dereferencing this pointer after converting it to be a pointer to the target type.

53.  The manner of choosing a storage pool for an access type when Storage_Pool is not specified for the type.  See 13.11(17).

There are 3 different standard pools used by the compiler when Storage_Pool is not specified depending whether the type is local to a subprogram or defined at the library level and whether Storage_Sizeis specified or not.  See documentation in the runtime library units System.Pool_Global, System.Pool_Size and System.Pool_Local in files s-poosiz.ads, s-pooglo.ads and s-pooloc.ads for full details on the default pools used.

54.  Whether or not the implementation provides user-accessible names for the standard pool type(s).  See 13.11(17).

See documentation in the sources of the run time mentioned in paragraph 53 .  All these pools are accessible by means of with'ing these units.

55.  The meaning of Storage_Size.  See 13.11(18).

Storage_Size is measured in storage units, and refers to the total space available for an access type collection, or to the primary stack space for a task.

56.  Implementation-defined aspects of storage pools.  See 13.11(22).

See documentation in the sources of the run time mentioned in paragraph 53 for details on GNAT-defined aspects of storage pools.

57.  The set of restrictions allowed in a pragma Restrictions.  See 13.12(7).

All RM defined Restriction identifiers are implemented.  The following additional restriction identifiers are provided.  There are two separate lists of implementation dependent restriction identifiers.  The first set requires consistency throughout a partition (in other words, if the restriction identifier is used for any compilation unit in the partition, then all compilation units in the partition must obey the restriction.

Simple_Barriers
     This restriction ensures at compile time that barriers in entry declarations for protected types are restricted to either static boolean expressions or references to simple boolean variables defined in the private part of the protected type.  No other form of entry barriers is permitted.  This is one of the restrictions of the Ravenscar profile for limited tasking (see also pragma Profile (Ravenscar)).

Max_Entry_Queue_Length => Expr
     This restriction is a declaration that any protected entry compiled in the scope of the restriction has at most the specified number of tasks waiting on the entry at any one time, and so no queue is required.  This restriction is not checked at compile time.  A program execution is erroneous if an attempt is made to queue more than the specified number of tasks on such an entry.

No_Calendar
     This restriction ensures at compile time that there is no implicit or explicit dependence on the package Ada.Calendar.

No_Direct_Boolean_Operators
     This restriction ensures that no logical (and/or/xor) or comparison operators are used on operands of type Boolean (or any type derived from Boolean). This is intended for use in safety critical programs where the certification protocol requires the use of short-circuit (and then, or else) forms for all composite boolean operations.

No_Dynamic_Attachment
     This restriction ensures that there is no call to any of the operations defined in package Ada.Interrupts.

No_Enumeration_Maps
     This restriction ensures at compile time that no operations requiring enumeration maps are used (that is Image and Value attributes applied to enumeration types).

No_Entry_Calls_In_Elaboration_Code
     This restriction ensures at compile time that no task or protected entry calls are made during elaboration code.  As a result of the use of this restriction, the compiler can assume that no code past an accept statement in a task can be executed at elaboration time.

No_Exception_Handlers
     This restriction ensures at compile time that there are no explicit exception handlers. It also indicates that no exception propagation will be provided. In this mode, exceptions may be raised but will result in an immediate call to the last chance handler, a routine that the user must define with the following profile:

     procedure Last_Chance_Handler (Source_Location : System.Address; Line : Integer); pragma Export (C, Last_Chance_Handler, "__gnat_last_chance_handler");

     The parameter is a C null-terminated string representing a message to be associated with the exception (typically the source location of the raise statement generated by the compiler). The Line parameter when non-zero represents the line number in the source program where the raise occurs.

No_Exception_Streams
     This restriction ensures at compile time that no stream operations for types Exception_Id or Exception_Occurrence are used. This also makes it impossible to pass exceptions to or from a partition with this restriction in a distributed environment. If this exception is active, then the generated code is simplified by omitting the otherwise-required global registration of exceptions when they are declared.

No_Implicit_Conditionals
     This restriction ensures that the generated code does not contain any implicit conditionals, either by modifying the generated code where possible, or by rejecting any construct that would otherwise generate an implicit conditional. Note that this check does not include run time constraint checks, which on some targets may generate implicit conditionals as well. To control the latter, constraint checks can be suppressed in the normal manner.

No_Implicit_Dynamic_Code
     This restriction prevents the compiler from building ``trampolines''. This is a structure that is built on the stack and contains dynamic code to be executed at run time. A trampoline is needed to indirectly address a nested subprogram (that is a subprogram that is not at the library level). The restriction prevents the use of any of the attributes Address, Access or Unrestricted_Access being applied to a subprogram that is not at the library level.

No_Implicit_Loops
     This restriction ensures that the generated code does not contain any implicit for loops, either by modifying the generated code where possible, or by rejecting any construct that would otherwise generate an implicit for loop.

No_Initialize_Scalars
     This restriction ensures that no unit in the partition is compiled with pragma Initialize_Scalars. This allows the generation of more efficient code, and in particular eliminates dummy null initialization routines that are otherwise generated for some record and array types.

No_Local_Protected_Objects
     This restriction ensures at compile time that protected objects are only declared at the library level.

No_Protected_Type_Allocators
     This restriction ensures at compile time that there are no allocator expressions that attempt to allocate protected objects.

No_Secondary_Stack
     This restriction ensures at compile time that the generated code does not contain any reference to the secondary stack.  The secondary stack is used to implement functions returning unconstrained objects (arrays or records) on some targets.

No_Select_Statements
     This restriction ensures at compile time no select statements of any kind are permitted, that is the keyword select may not appear. This is one of the restrictions of the Ravenscar profile for limited tasking (see also pragma Profile (Ravenscar)).

No_Standard_Storage_Pools
     This restriction ensures at compile time that no access types use the standard default storage pool.  Any access type declared must have an explicit Storage_Pool attribute defined specifying a user-defined storage pool.

No_Streams
     This restriction ensures at compile/bind time that there are no stream objects created (and therefore no actual stream operations). This restriction does not forbid dependences on the package Ada.Streams. So it is permissible to with Ada.Streams (or another package that does so itself) as long as no actual stream objects are created.

No_Task_Attributes_Package
     This restriction ensures at compile time that there are no implicit or explicit dependencies on the package Ada.Task_Attributes.

No_Task_Termination
     This restriction ensures at compile time that no terminate alternatives appear in any task body.

No_Tasking
     This restriction prevents the declaration of tasks or task types throughout the partition.  It is similar in effect to the use of Max_Tasks => 0 except that violations are caught at compile time and cause an error message to be output either by the compiler or binder.

No_Wide_Characters
     This restriction ensures at compile time that no uses of the types Wide_Character or Wide_String or corresponding wide wide types appear, and that no wide or wide wide string or character literals appear in the program (that is literals representing characters not in type Character.

Static_Priorities
     This restriction ensures at compile time that all priority expressions are static, and that there are no dependencies on the package Ada.Dynamic_Priorities.

Static_Storage_Size
     This restriction ensures at compile time that any expression appearing in a Storage_Size pragma or attribute definition clause is static.

The second set of implementation dependent restriction identifiers does not require partition-wide consistency. The restriction may be enforced for a single compilation unit without any effect on any of the other compilation units in the partition.

No_Elaboration_Code
     This restriction ensures at compile time that no elaboration code is generated.  Note that this is not the same condition as is enforced by pragma Preelaborate.  There are cases in which pragma Preelaborate still permits code to be generated (e.g. code to initialize a large array to all zeroes), and there are cases of units which do not meet the requirements for pragma Preelaborate, but for which no elaboration code is generated.  Generally, it is the case that preelaborable units will meet the restrictions, with the exception of large aggregates initialized with an others_clause, and exception declarations (which generate calls to a run-time registry procedure).  Note that this restriction is enforced on a unit by unit basis, it need not be obeyed consistently throughout a partition.

No_Entry_Queue
     This restriction is a declaration that any protected entry compiled in the scope of the restriction has at most one task waiting on the entry at any one time, and so no queue is required.  This restriction is not checked at compile time.  A program execution is erroneous if an attempt is made to queue a second task on such an entry.

No_Implementation_Attributes
     This restriction checks at compile time that no GNAT-defined attributes are present.  With this restriction, the only attributes that can be used are those defined in the Ada 95 Reference Manual.

No_Implementation_Pragmas
     This restriction checks at compile time that no GNAT-defined pragmas are present.  With this restriction, the only pragmas that can be used are those defined in the Ada 95 Reference Manual.

No_Implementation_Restrictions
     This restriction checks at compile time that no GNAT-defined restriction identifiers (other than No_Implementation_Restrictions itself) are present.  With this restriction, the only other restriction identifiers that can be used are those defined in the Ada 95 Reference Manual.

58.  The consequences of violating limitations on Restrictions pragmas.  See 13.12(9).

Restrictions that can be checked at compile time result in illegalities if violated.  Currently there are no other consequences of violating restrictions.

59.  The representation used by the Read and Write attributes of elementary types in terms of stream elements.  See 13.13.2(9).

The representation is the in-memory representation of the base type of the type, using the number of bits corresponding to the TYPE'Size value, and the natural ordering of the machine.

60.  The names and characteristics of the numeric subtypes declared in the visible part of package Standard.  See A.1(3).

See items describing the integer and floating-point types supported.

61.  The accuracy actually achieved by the elementary functions.  See A.5.1(1).

The elementary functions correspond to the functions available in the C library.  Only fast math mode is implemented.

62.  The sign of a zero result from some of the operators or functions in Numerics.Generic_Elementary_Functions, when Float_Type'Signed_Zeros is True.  See A.5.1(46).

The sign of zeroes follows the requirements of the IEEE 754 standard on floating-point.

63.  The value of Numerics.Float_Random.Max_Image_Width.  See A.5.2(27).

Maximum image width is 649, see library file a-numran.ads.

64.  The value of Numerics.Discrete_Random.Max_Image_Width.  See A.5.2(27).

Maximum image width is 80, see library file a-nudira.ads.

65.  The algorithms for random number generation.  See A.5.2(32).

The algorithm is documented in the source files a-numran.ads and a-numran.adb.

66.  The string representation of a random number generator's state.  See A.5.2(38).

See the documentation contained in the file a-numran.adb.

67.  The minimum time interval between calls to the time-dependent Reset procedure that are guaranteed to initiate different random number sequences.  See A.5.2(45).

The minimum period between reset calls to guarantee distinct series of random numbers is one microsecond.

68.  The values of the Model_Mantissa, Model_Emin, Model_Epsilon, Model, Safe_First, and Safe_Last attributes, if the Numerics Annex is not supported.  See A.5.3(72).

See the source file ttypef.ads for the values of all numeric attributes.

69.  Any implementation-defined characteristics of the input-output packages.  See A.7(14).

There are no special implementation defined characteristics for these packages.

70.  The value of Buffer_Size in Storage_IO.  See A.9(10).

All type representations are contiguous, and the Buffer_Size is the value of TYPE'Size rounded up to the next storage unit boundary.

71.  External files for standard input, standard output, and standard error See A.10(5).

These files are mapped onto the files provided by the C streams libraries.  See source file i-cstrea.ads for further details.

72.  The accuracy of the value produced by Put.  See A.10.9(36).

If more digits are requested in the output than are represented by the precision of the value, zeroes are output in the corresponding least significant digit positions.

73.  The meaning of Argument_Count, Argument, and Command_Name.  See A.15(1).

These are mapped onto the argv and argc parameters of the main program in the natural manner.

74.  Implementation-defined convention names.  See B.1(11).

The following convention names are supported

Ada
     Ada

Assembler
     Assembly language

Asm
     Synonym for Assembler

Assembly
     Synonym for Assembler

C
     C

C_Pass_By_Copy
     Allowed only for record types, like C, but also notes that record is to be passed by copy rather than reference.

COBOL
     COBOL

CPP
     C++

Default
     Treated the same as C

External
     Treated the same as C

Fortran
     Fortran

Intrinsic
     For support of pragma Import with convention Intrinsic, see separate section on Intrinsic Subprograms.

Stdcall
     Stdcall (used for Windows implementations only).  This convention correspond to the WINAPI (previously called Pascal convention) C/C++ convention under Windows.  A function with this convention cleans the stack before exit.

DLL
     Synonym for Stdcall

Win32
     Synonym for Stdcall

Stubbed
     Stubbed is a special convention used to indicate that the body of the subprogram will be entirely ignored.  Any call to the subprogram is converted into a raise of the Program_Error exception.  If a pragma Import specifies convention stubbed then no body need be present at all.  This convention is useful during development for the inclusion of subprograms whose body has not yet been written.

In addition, all otherwise unrecognized convention names are also treated as being synonymous with convention C.  In all implementations except for VMS, use of such other names results in a warning.  In VMS implementations, these names are accepted silently.

75.  The meaning of link names.  See B.1(36).

Link names are the actual names used by the linker.

76.  The manner of choosing link names when neither the link name nor the address of an imported or exported entity is specified.  See B.1(36).

The default linker name is that which would be assigned by the relevant external language, interpreting the Ada name as being in all lower case letters.

77.  The effect of pragma Linker_Options.  See B.1(37).

The string passed to Linker_Options is presented uninterpreted as an argument to the link command, unless it contains Ascii.NUL characters. NUL characters if they appear act as argument separators, so for example

     pragma Linker_Options ("-labc" & ASCII.Nul & "-ldef");

causes two separate arguments -labc and -ldef to be passed to the linker. The order of linker options is preserved for a given unit. The final list of options passed to the linker is in reverse order of the elaboration order. For example, linker options fo a body always appear before the options from the corresponding package spec.

78.  The contents of the visible part of package Interfaces and its language-defined descendants.  See B.2(1).

See files with prefix i- in the distributed library.

79.  Implementation-defined children of package Interfaces.  The contents of the visible part of package Interfaces.  See B.2(11).

See files with prefix i- in the distributed library.

80.  The types Floating, Long_Floating, Binary, Long_Binary, Decimal_ Element, and COBOL_Character; and the initialization of the variables Ada_To_COBOL and COBOL_To_Ada, in Interfaces.COBOL.  See B.4(50).

Floating
     Float

Long_Floating
     (Floating) Long_Float

Binary
     Integer

Long_Binary
     Long_Long_Integer

Decimal_Element
     Character

COBOL_Character
     Character

For initialization, see the file i-cobol.ads in the distributed library.

81.  Support for access to machine instructions.  See C.1(1).

See documentation in file s-maccod.ads in the distributed library.

82.  Implementation-defined aspects of access to machine operations.  See C.1(9).

See documentation in file s-maccod.ads in the distributed library.

83.  Implementation-defined aspects of interrupts.  See C.3(2).

Interrupts are mapped to signals or conditions as appropriate.  See definition of unit Ada.Interrupt_Names in source file a-intnam.ads for details on the interrupts supported on a particular target.

84.  Implementation-defined aspects of pre-elaboration.  See C.4(13).

GNAT does not permit a partition to be restarted without reloading, except under control of the debugger.

85.  The semantics of pragma Discard_Names.  See C.5(7).

Pragma Discard_Names causes names of enumeration literals to be suppressed.  In the presence of this pragma, the Image attribute provides the image of the Pos of the literal, and Value accepts Pos values.

86.  The result of the Task_Identification.Image attribute.  See C.7.1(7).

The result of this attribute is an 8-digit hexadecimal string representing the virtual address of the task control block.

87.  The value of Current_Task when in a protected entry or interrupt handler.  See C.7.1(17).

Protected entries or interrupt handlers can be executed by any convenient thread, so the value of Current_Task is undefined.

88.  The effect of calling Current_Task from an entry body or interrupt handler.  See C.7.1(19).

The effect of calling Current_Task from an entry body or interrupt handler is to return the identification of the task currently executing the code.

89.  Implementation-defined aspects of Task_Attributes.  See C.7.2(19).

There are no implementation-defined aspects of Task_Attributes.

90.  Values of all Metrics.  See D(2).

The metrics information for GNAT depends on the performance of the underlying operating system.  The sources of the run-time for tasking implementation, together with the output from -gnatG can be used to determine the exact sequence of operating systems calls made to implement various tasking constructs.  Together with appropriate information on the performance of the underlying operating system, on the exact target in use, this information can be used to determine the required metrics.

91.  The declarations of Any_Priority and Priority.  See D.1(11).

See declarations in file system.ads.

92.  Implementation-defined execution resources.  See D.1(15).

There are no implementation-defined execution resources.

93.  Whether, on a multiprocessor, a task that is waiting for access to a protected object keeps its processor busy.  See D.2.1(3).

On a multi-processor, a task that is waiting for access to a protected object does not keep its processor busy.

94.  The affect of implementation defined execution resources on task dispatching.  See D.2.1(9).

Tasks map to threads in the threads package used by GNAT.  Where possible and appropriate, these threads correspond to native threads of the underlying operating system.

95.  Implementation-defined policy_identifiers allowed in a pragma Task_Dispatching_Policy.  See D.2.2(3).

There are no implementation-defined policy-identifiers allowed in this pragma.

96.  Implementation-defined aspects of priority inversion.  See D.2.2(16).

Execution of a task cannot be preempted by the implementation processing of delay expirations for lower priority tasks.

97.  Implementation defined task dispatching.  See D.2.2(18).

The policy is the same as that of the underlying threads implementation.

98.  Implementation-defined policy_identifiers allowed in a pragma Locking_Policy.  See D.3(4).

The only implementation defined policy permitted in GNAT is Inheritance_Locking.  On targets that support this policy, locking is implemented by inheritance, i.e. the task owning the lock operates at a priority equal to the highest priority of any task currently requesting the lock.

99.  Default ceiling priorities.  See D.3(10).

The ceiling priority of protected objects of the type System.Interrupt_Priority'Last as described in the Ada 95 Reference Manual D.3(10),

100.  The ceiling of any protected object used internally by the implementation.  See D.3(16).

The ceiling priority of internal protected objects is System.Priority'Last.

101.  Implementation-defined queuing policies.  See D.4(1).

There are no implementation-defined queueing policies.

102.  On a multiprocessor, any conditions that cause the completion of an aborted construct to be delayed later than what is specified for a single processor.  See D.6(3).

The semantics for abort on a multi-processor is the same as on a single processor, there are no further delays.

103.  Any operations that implicitly require heap storage allocation.  See D.7(8).

The only operation that implicitly requires heap storage allocation is task creation.

104.  Implementation-defined aspects of pragma Restrictions.  See D.7(20).

There are no such implementation-defined aspects.

105.  Implementation-defined aspects of package Real_Time.  See D.8(17).

There are no implementation defined aspects of package Real_Time.

106.  Implementation-defined aspects of delay_statements.  See D.9(8).

Any difference greater than one microsecond will cause the task to be delayed (see D.9(7)).

107.  The upper bound on the duration of interrupt blocking caused by the implementation.  See D.12(5).

The upper bound is determined by the underlying operating system.  In no cases is it more than 10 milliseconds.

108.  The means for creating and executing distributed programs.  See E(5).

The GLADE package provides a utility GNATDIST for creating and executing distributed programs.  See the GLADE reference manual for further details.

109.  Any events that can result in a partition becoming inaccessible.  See E.1(7).

See the GLADE reference manual for full details on such events.

110.  The scheduling policies, treatment of priorities, and management of shared resources between partitions in certain cases.  See E.1(11).

See the GLADE reference manual for full details on these aspects of multi-partition execution.

111.  Events that cause the version of a compilation unit to change.  See E.3(5).

Editing the source file of a compilation unit, or the source files of any units on which it is dependent in a significant way cause the version to change.  No other actions cause the version number to change.  All changes are significant except those which affect only layout, capitalization or comments.

112.  Whether the execution of the remote subprogram is immediately aborted as a result of cancellation.  See E.4(13).

See the GLADE reference manual for details on the effect of abort in a distributed application.

113.  Implementation-defined aspects of the PCS.  See E.5(25).

See the GLADE reference manual for a full description of all implementation defined aspects of the PCS.

114.  Implementation-defined interfaces in the PCS.  See E.5(26).

See the GLADE reference manual for a full description of all implementation defined interfaces.

115.  The values of named numbers in the package Decimal.  See F.2(7).

Max_Scale
     +18

Min_Scale
     -18

Min_Delta
     1.0E-18

Max_Delta
     1.0E+18

Max_Decimal_Digits
     18

116.  The value of Max_Picture_Length in the package Text_IO.Editing.  See F.3.3(16).

64

117.  The value of Max_Picture_Length in the package Wide_Text_IO.Editing.  See F.3.4(5).

64

118.  The accuracy actually achieved by the complex elementary functions and by other complex arithmetic operations.  See G.1(1).

Standard library functions are used for the complex arithmetic operations.  Only fast math mode is currently supported.

119.  The sign of a zero result (or a component thereof) from any operator or function in Numerics.Generic_Complex_Types, when Real'Signed_Zeros is True.  See G.1.1(53).

The signs of zero values are as recommended by the relevant implementation advice.

120.  The sign of a zero result (or a component thereof) from any operator or function in Numerics.Generic_Complex_Elementary_Functions, when Real'Signed_Zeros is True.  See G.1.2(45).

The signs of zero values are as recommended by the relevant implementation advice.

121.  Whether the strict mode or the relaxed mode is the default.  See G.2(2).

The strict mode is the default.  There is no separate relaxed mode.  GNAT provides a highly efficient implementation of strict mode.

122.  The result interval in certain cases of fixed-to-float conversion.  See G.2.1(10).

For cases where the result interval is implementation dependent, the accuracy is that provided by performing all operations in 64-bit IEEE floating-point format.

123.  The result of a floating point arithmetic operation in overflow situations, when the Machine_Overflows attribute of the result type is False.  See G.2.1(13).

Infinite and Nan values are produced as dictated by the IEEE floating-point standard.

124.  The result interval for division (or exponentiation by a negative exponent), when the floating point hardware implements division as multiplication by a reciprocal.  See G.2.1(16).

Not relevant, division is IEEE exact.

125.  The definition of close result set, which determines the accuracy of certain fixed point multiplications and divisions.  See G.2.3(5).

Operations in the close result set are performed using IEEE long format floating-point arithmetic.  The input operands are converted to floating-point, the operation is done in floating-point, and the result is converted to the target type.

126.  Conditions on a universal_real operand of a fixed point multiplication or division for which the result shall be in the perfect result set.  See G.2.3(22).

The result is only defined to be in the perfect result set if the result can be computed by a single scaling operation involving a scale factor representable in 64-bits.

127.  The result of a fixed point arithmetic operation in overflow situations, when the Machine_Overflows attribute of the result type is False.  See G.2.3(27).

Not relevant, Machine_Overflows is True for fixed-point types.

128.  The result of an elementary function reference in overflow situations, when the Machine_Overflows attribute of the result type is False.  See G.2.4(4).

IEEE infinite and Nan values are produced as appropriate.

129.  The value of the angle threshold, within which certain elementary functions, complex arithmetic operations, and complex elementary functions yield results conforming to a maximum relative error bound.  See G.2.4(10).

Information on this subject is not yet available.

130.  The accuracy of certain elementary functions for parameters beyond the angle threshold.  See G.2.4(10).

Information on this subject is not yet available.

131.  The result of a complex arithmetic operation or complex elementary function reference in overflow situations, when the Machine_Overflows attribute of the corresponding real type is False.  See G.2.6(5).

IEEE infinite and Nan values are produced as appropriate.

132.  The accuracy of certain complex arithmetic operations and certain complex elementary functions for parameters (or components thereof) beyond the angle threshold.  See G.2.6(8).

Information on those subjects is not yet available.

133.  Information regarding bounded errors and erroneous execution.  See H.2(1).

Information on this subject is not yet available.

134.  Implementation-defined aspects of pragma Inspection_Point.  See H.3.2(8).

Pragma Inspection_Point ensures that the variable is live and can be examined by the debugger at the inspection point.

135.  Implementation-defined aspects of pragma Restrictions.  See H.4(25).

There are no implementation-defined aspects of pragma Restrictions.  The use of pragma Restrictions [No_Exceptions] has no effect on the generated code.  Checks must suppressed by use of pragma Suppress.

136.  Any restrictions on pragma Restrictions.  See H.4(27).

There are no restrictions on pragma Restrictions.

Intrinsic Subprograms
*********************

GNAT allows a user application program to write the declaration:

        pragma Import (Intrinsic, name);

providing that the name corresponds to one of the implemented intrinsic subprograms in GNAT, and that the parameter profile of the referenced subprogram meets the requirements.  This chapter describes the set of implemented intrinsic subprograms, and the requirements on parameter profiles. Note that no body is supplied; as with other uses of pragma Import, the body is supplied elsewhere (in this case by the compiler itself).  Note that any use of this feature is potentially non-portable, since the Ada standard does not require Ada compilers to implement this feature.

Intrinsic Operators
===================

All the predefined numeric operators in package Standard in pragma Import (Intrinsic,..) declarations.  In the binary operator case, the operands must have the same size.  The operand or operands must also be appropriate for the operator.  For example, for addition, the operands must both be floating-point or both be fixed-point, and the right operand for "**" must have a root type of Standard.Integer'Base. You can use an intrinsic operator declaration as in the following example:

        type Int1 is new Integer;
        type Int2 is new Integer;

        function "+" (X1 : Int1; X2 : Int2) return Int1;
        function "+" (X1 : Int1; X2 : Int2) return Int2;
        pragma Import (Intrinsic, "+");

This declaration would permit ``mixed mode'' arithmetic on items of the differing types Int1 and Int2. It is also possible to specify such operators for private types, if the full views are appropriate arithmetic types.

Enclosing_Entity
================

This intrinsic subprogram is used in the implementation of the library routine GNAT.Source_Info.  The only useful use of the intrinsic import in this case is the one in this unit, so an application program should simply call the function GNAT.Source_Info.Enclosing_Entity to obtain the name of the current subprogram, package, task, entry, or protected subprogram.

Exception_Information
=====================

This intrinsic subprogram is used in the implementation of the library routine GNAT.Current_Exception.  The only useful use of the intrinsic import in this case is the one in this unit, so an application program should simply call the function GNAT.Current_Exception.Exception_Information to obtain the exception information associated with the current exception.

Exception_Message
=================

This intrinsic subprogram is used in the implementation of the library routine GNAT.Current_Exception.  The only useful use of the intrinsic import in this case is the one in this unit, so an application program should simply call the function GNAT.Current_Exception.Exception_Message to obtain the message associated with the current exception.

Exception_Name
==============

This intrinsic subprogram is used in the implementation of the library routine GNAT.Current_Exception.  The only useful use of the intrinsic import in this case is the one in this unit, so an application program should simply call the function GNAT.Current_Exception.Exception_Name to obtain the name of the current exception.

File
====

This intrinsic subprogram is used in the implementation of the library routine GNAT.Source_Info.  The only useful use of the intrinsic import in this case is the one in this unit, so an application program should simply call the function GNAT.Source_Info.File to obtain the name of the current file.

Line
====

This intrinsic subprogram is used in the implementation of the library routine GNAT.Source_Info.  The only useful use of the intrinsic import in this case is the one in this unit, so an application program should simply call the function GNAT.Source_Info.Line to obtain the number of the current source line.

Rotate_Left
===========

In standard Ada 95, the Rotate_Left function is available only for the predefined modular types in package Interfaces.  However, in GNAT it is possible to define a Rotate_Left function for a user defined modular type or any signed integer type as in this example:

        function Shift_Left
          (Value  : My_Modular_Type;
           Amount : Natural)
           return   My_Modular_Type;

The requirements are that the profile be exactly as in the example above.  The only modifications allowed are in the formal parameter names, and in the type of Value and the return type, which must be the same, and must be either a signed integer type, or a modular integer type with a binary modulus, and the size must be 8.  16, 32 or 64 bits.

Rotate_Right
============

A Rotate_Right function can be defined for any user defined binary modular integer type, or signed integer type, as described above for Rotate_Left.

Shift_Left
==========

A Shift_Left function can be defined for any user defined binary modular integer type, or signed integer type, as described above for Rotate_Left.

Shift_Right
===========

A Shift_Right function can be defined for any user defined binary modular integer type, or signed integer type, as described above for Rotate_Left.

Shift_Right_Arithmetic
======================

A Shift_Right_Arithmetic function can be defined for any user defined binary modular integer type, or signed integer type, as described above for Rotate_Left.

Source_Location
===============

This intrinsic subprogram is used in the implementation of the library routine GNAT.Source_Info.  The only useful use of the intrinsic import in this case is the one in this unit, so an application program should simply call the function GNAT.Source_Info.Source_Location to obtain the current source file location.

Representation Clauses and Pragmas
**********************************

This section describes the representation clauses accepted by GNAT, and their effect on the representation of corresponding data objects.

GNAT fully implements Annex C (Systems Programming).  This means that all the implementation advice sections in chapter 13 are fully implemented. However, these sections only require a minimal level of support for representation clauses.  GNAT provides much more extensive capabilities, and this section describes the additional capabilities provided.

Alignment Clauses
=================

GNAT requires that all alignment clauses specify a power of 2, and all default alignments are always a power of 2.  The default alignment values are as follows:

  * Primitive Types. For primitive types, the alignment is the minimum of the actual size of objects of the type divided by Storage_Unit, and the maximum alignment supported by the target. (This maximum alignment is given by the GNAT-specific attribute Standard'Maximum_Alignment; see Maximum_Alignment.) For example, for type Long_Float, the object size is 8 bytes, and the default alignment will be 8 on any target that supports alignments this large, but on some targets, the maximum alignment may be smaller than 8, in which case objects of type Long_Float will be maximally aligned.

  * Arrays. For arrays, the alignment is equal to the alignment of the component type for the normal case where no packing or component size is given.  If the array is packed, and the packing is effective (see separate section on packed arrays), then the alignment will be one for long packed arrays, or arrays whose length is not known at compile time.  For short packed arrays, which are handled internally as modular types, the alignment will be as described for primitive types, e.g. a packed array of length 31 bits will have an object size of four bytes, and an alignment of 4.

  * Records. For the normal non-packed case, the alignment of a record is equal to the maximum alignment of any of its components.  For tagged records, this includes the implicit access type used for the tag.  If a pragma Pack is used and all fields are packable (see separate section on pragma Pack), then the resulting alignment is 1.

     A special case is when:

       * the size of the record is given explicitly, or a full record representation clause is given, and

       * the size of the record is 2, 4, or 8 bytes.

     In this case, an alignment is chosen to match the size of the record. For example, if we have:

             type Small is record
                A, B : Character;
             end record;
             for Small'Size use 16;

     then the default alignment of the record type Small is 2, not 1. This leads to more efficient code when the record is treated as a unit, and also allows the type to specified as Atomic on architectures requiring strict alignment.

An alignment clause may always specify a larger alignment than the default value, up to some maximum value dependent on the target (obtainable by using the attribute reference Standard'Maximum_Alignment). The only case where it is permissible to specify a smaller alignment than the default value is for a record with a record representation clause. In this case, packable fields for which a component clause is given still result in a default alignment corresponding to the original type, but this may be overridden, since these components in fact only require an alignment of one byte.  For example, given

       type V is record
          A : Integer;
       end record;

       for V use record
          A at 0  range 0 .. 31;
       end record;

       for V'alignment use 1;

The default alignment for the type V is 4, as a result of the Integer field in the record, but since this field is placed with a component clause, it is permissible, as shown, to override the default alignment of the record with a smaller value.

Size Clauses
============

The default size for a type T is obtainable through the language-defined attribute T'Size and also through the equivalent GNAT-defined attribute T'Value_Size. For objects of type T, GNAT will generally increase the type size so that the object size (obtainable through the GNAT-defined attribute T'Object_Size) is a multiple of T'Alignment * Storage_Unit. For example

        type Smallint is range 1 .. 6;

        type Rec is record
           Y1 : integer;
           Y2 : boolean;
        end record;

In this example, Smallint'Size = Smallint'Value_Size = 3, as specified by the RM rules, but objects of this type will have a size of 8 (Smallint'Object_Size = 8), since objects by default occupy an integral number of storage units.  On some targets, notably older versions of the Digital Alpha, the size of stand alone objects of this type may be 32, reflecting the inability of the hardware to do byte load/stores.

Similarly, the size of type Rec is 40 bits (Rec'Size = Rec'Value_Size = 40), but the alignment is 4, so objects of this type will have their size increased to 64 bits so that it is a multiple of the alignment (in bits).  This decision is in accordance with the specific Implementation Advice in RM 13.3(43):

     A Size clause should be supported for an object if the specified Size is at least as large as its subtype's Size, and corresponds to a size in storage elements that is a multiple of the object's Alignment (if the Alignment is nonzero).

An explicit size clause may be used to override the default size by increasing it.  For example, if we have:

        type My_Boolean is new Boolean;
        for My_Boolean'Size use 32;

then values of this type will always be 32 bits long.  In the case of discrete types, the size can be increased up to 64 bits, with the effect that the entire specified field is used to hold the value, sign- or zero-extended as appropriate.  If more than 64 bits is specified, then padding space is allocated after the value, and a warning is issued that there are unused bits.

Similarly the size of records and arrays may be increased, and the effect is to add padding bits after the value.  This also causes a warning message to be generated.

The largest Size value permitted in GNAT is 2**31-1.  Since this is a Size in bits, this corresponds to an object of size 256 megabytes (minus one).  This limitation is true on all targets.  The reason for this limitation is that it improves the quality of the code in many cases if it is known that a Size value can be accommodated in an object of type Integer.

Storage_Size Clauses
====================

For tasks, the Storage_Size clause specifies the amount of space to be allocated for the task stack.  This cannot be extended, and if the stack is exhausted, then Storage_Error will be raised (if stack checking is enabled).  Use a Storage_Size attribute definition clause, or a Storage_Size pragma in the task definition to set the appropriate required size.  A useful technique is to include in every task definition a pragma of the form:

        pragma Storage_Size (Default_Stack_Size);

Then Default_Stack_Size can be defined in a global package, and modified as required.  Any tasks requiring stack sizes different from the default can have an appropriate alternative reference in the pragma.

For access types, the Storage_Size clause specifies the maximum space available for allocation of objects of the type.  If this space is exceeded then Storage_Error will be raised by an allocation attempt. In the case where the access type is declared local to a subprogram, the use of a Storage_Size clause triggers automatic use of a special predefined storage pool (System.Pool_Size) that ensures that all space for the pool is automatically reclaimed on exit from the scope in which the type is declared.

A special case recognized by the compiler is the specification of a Storage_Size of zero for an access type.  This means that no items can be allocated from the pool, and this is recognized at compile time, and all the overhead normally associated with maintaining a fixed size storage pool is eliminated.  Consider the following example:

        procedure p is
           type R is array (Natural) of Character;
           type P is access all R;
           for P'Storage_Size use 0;
           --  Above access type intended only for interfacing purposes

           y : P;

           procedure g (m : P);
           pragma Import (C, g);

           --  ...

        begin
           --  ...
           y := new R;
        end;

As indicated in this example, these dummy storage pools are often useful in connection with interfacing where no object will ever be allocated.  If you compile the above example, you get the warning:

        p.adb:16:09: warning: allocation from empty storage pool
        p.adb:16:09: warning: Storage_Error will be raised at run time

Of course in practice, there will not be any explicit allocators in the case of such an access declaration.

Size of Variant Record Objects
==============================

In the case of variant record objects, there is a question whether Size gives information about a particular variant, or the maximum size required for any variant.  Consider the following program

     with Text_IO; use Text_IO;
     procedure q is
        type R1 (A : Boolean := False) is record
          case A is
            when True  => X : Character;
            when False => null;
          end case;
        end record;

        V1 : R1 (False);
        V2 : R1;

     begin
        Put_Line (Integer'Image (V1'Size));
        Put_Line (Integer'Image (V2'Size));
     end q;

Here we are dealing with a variant record, where the True variant requires 16 bits, and the False variant requires 8 bits. In the above example, both V1 and V2 contain the False variant, which is only 8 bits long.  However, the result of running the program is:

     8
     16

The reason for the difference here is that the discriminant value of V1 is fixed, and will always be False.  It is not possible to assign a True variant value to V1, therefore 8 bits is sufficient.  On the other hand, in the case of V2, the initial discriminant value is False (from the default), but it is possible to assign a True variant value to V2, therefore 16 bits must be allocated for V2 in the general case, even fewer bits may be needed at any particular point during the program execution.

As can be seen from the output of this program, the 'Size attribute applied to such an object in GNAT gives the actual allocated size of the variable, which is the largest size of any of the variants. The Ada Reference Manual is not completely clear on what choice should be made here, but the GNAT behavior seems most consistent with the language in the RM.

In some cases, it may be desirable to obtain the size of the current variant, rather than the size of the largest variant.  This can be achieved in GNAT by making use of the fact that in the case of a subprogram parameter, GNAT does indeed return the size of the current variant (because a subprogram has no way of knowing how much space is actually allocated for the actual).

Consider the following modified version of the above program:

     with Text_IO; use Text_IO;
     procedure q is
        type R1 (A : Boolean := False) is record
          case A is
            when True  => X : Character;
            when False => null;
          end case;
        end record;

        V2 : R1;

        function Size (V : R1) return Integer is
        begin
           return V'Size;
        end Size;

     begin
        Put_Line (Integer'Image (V2'Size));
        Put_Line (Integer'IMage (Size (V2)));
        V2 := (True, 'x');
        Put_Line (Integer'Image (V2'Size));
        Put_Line (Integer'IMage (Size (V2)));
     end q;

The output from this program is

     16
     8
     16
     16

Here we see that while the 'Size attribute always returns the maximum size, regardless of the current variant value, the Size function does indeed return the size of the current variant value.

Biased Representation
=====================

In the case of scalars with a range starting at other than zero, it is possible in some cases to specify a size smaller than the default minimum value, and in such cases, GNAT uses an unsigned biased representation, in which zero is used to represent the lower bound, and successive values represent successive values of the type.

For example, suppose we have the declaration:

        type Small is range -7 .. -4;
        for Small'Size use 2;

Although the default size of type Small is 4, the Size clause is accepted by GNAT and results in the following representation scheme:

       -7 is represented as 2#00#
       -6 is represented as 2#01#
       -5 is represented as 2#10#
       -4 is represented as 2#11#

Biased representation is only used if the specified Size clause cannot be accepted in any other manner.  These reduced sizes that force biased representation can be used for all discrete types except for enumeration types for which a representation clause is given.

Value_Size and Object_Size Clauses
==================================

In Ada 95, T'Size for a type T is the minimum number of bits required to hold values of type T.  Although this interpretation was allowed in Ada 83, it was not required, and this requirement in practice can cause some significant difficulties.  For example, in most Ada 83 compilers, Natural'Size was 32.  However, in Ada 95, Natural'Size is typically 31.  This means that code may change in behavior when moving from Ada 83 to Ada 95.  For example, consider:

        type Rec is record;
           A : Natural;
           B : Natural;
        end record;

        for Rec use record
           at 0  range 0 .. Natural'Size - 1;
           at 0  range Natural'Size .. 2 * Natural'Size - 1;
        end record;

In the above code, since the typical size of Natural objects is 32 bits and Natural'Size is 31, the above code can cause unexpected inefficient packing in Ada 95, and in general there are cases where the fact that the object size can exceed the size of the type causes surprises.

To help get around this problem GNAT provides two implementation defined attributes, Value_Size and Object_Size.  When applied to a type, these attributes yield the size of the type (corresponding to the RM defined size attribute), and the size of objects of the type respectively.

The Object_Size is used for determining the default size of objects and components.  This size value can be referred to using the Object_Size attribute.  The phrase ``is used'' here means that it is the basis of the determination of the size.  The backend is free to pad this up if necessary for efficiency, e.g. an 8-bit stand-alone character might be stored in 32 bits on a machine with no efficient byte access instructions such as the Alpha.

The default rules for the value of Object_Size for discrete types are as follows:

  * The Object_Size for base subtypes reflect the natural hardware size in bits (run the compiler with -gnatS to find those values for numeric types). Enumeration types and fixed-point base subtypes have 8, 16, 32 or 64 bits for this size, depending on the range of values to be stored.

  * The Object_Size of a subtype is the same as the Object_Size of the type from which it is obtained.

  * The Object_Size of a derived base type is copied from the parent base type, and the Object_Size of a derived first subtype is copied from the parent first subtype.

The Value_Size attribute is the (minimum) number of bits required to store a value of the type. This value is used to determine how tightly to pack records or arrays with components of this type, and also affects the semantics of unchecked conversion (unchecked conversions where the Value_Size values differ generate a warning, and are potentially target dependent).

The default rules for the value of Value_Size are as follows:

  * The Value_Size for a base subtype is the minimum number of bits required to store all values of the type (including the sign bit only if negative values are possible).

  * If a subtype statically matches the first subtype of a given type, then it has by default the same Value_Size as the first subtype.  This is a consequence of RM 13.1(14) (``if two subtypes statically match, then their subtype-specific aspects are the same''.)

  * All other subtypes have a Value_Size corresponding to the minimum number of bits required to store all values of the subtype.  For dynamic bounds, it is assumed that the value can range down or up to the corresponding bound of the ancestor

The RM defined attribute Size corresponds to the Value_Size attribute.

The Size attribute may be defined for a first-named subtype.  This sets the Value_Size of the first-named subtype to the given value, and the Object_Size of this first-named subtype to the given value padded up to an appropriate boundary.  It is a consequence of the default rules above that this Object_Size will apply to all further subtypes.  On the other hand, Value_Size is affected only for the first subtype, any dynamic subtypes obtained from it directly, and any statically matching subtypes.  The Value_Size of any other static subtypes is not affected.

Value_Size and Object_Size may be explicitly set for any subtype using an attribute definition clause.  Note that the use of these attributes can cause the RM 13.1(14) rule to be violated.  If two access types reference aliased objects whose subtypes have differing Object_Size values as a result of explicit attribute definition clauses, then it is erroneous to convert from one access subtype to the other.

At the implementation level, Esize stores the Object_Size and the RM_Size field stores the Value_Size (and hence the value of the Size attribute, which, as noted above, is equivalent to Value_Size).

To get a feel for the difference, consider the following examples (note that in each case the base is Short_Short_Integer with a size of 8):

                                            Object_Size     Value_Size

     type x1 is range 0 .. 5;                    8               3

     type x2 is range 0 .. 5;
     for x2'size use 12;                        16              12

     subtype x3 is x2 range 0 .. 3;             16               2

     subtype x4 is x2'base range 0 .. 10;        8               4

     subtype x5 is x2 range 0 .. dynamic;       16               3*

     subtype x6 is x2'base range 0 .. dynamic;   8               3*

Note: the entries marked ``3*'' are not actually specified by the Ada 95 RM, but it seems in the spirit of the RM rules to allocate the minimum number of bits (here 3, given the range for x2) known to be large enough to hold the given range of values.

So far, so good, but GNAT has to obey the RM rules, so the question is under what conditions must the RM Size be used. The following is a list of the occasions on which the RM Size must be used:

  * Component size for packed arrays or records

  * Value of the attribute Size for a type

  * Warning about sizes not matching for unchecked conversion

For record types, the Object_Size is always a multiple of the alignment of the type (this is true for all types). In some cases the Value_Size can be smaller. Consider:

        type R is record
          X : Integer;
          Y : Character;
        end record;

On a typical 32-bit architecture, the X component will be four bytes, and require four-byte alignment, and the Y component will be one byte. In this case R'Value_Size will be 40 (bits) since this is the minimum size required to store a value of this type, and for example, it is permissible to have a component of type R in an outer record whose component size is specified to be 48 bits. However, R'Object_Size will be 64 (bits), since it must be rounded up so that this value is a multiple of the alignment (4 bytes = 32 bits).

For all other types, the Object_Size and Value_Size are the same (and equivalent to the RM attribute Size). Only Size may be specified for such types.

Component_Size Clauses
======================

Normally, the value specified in a component clause must be consistent with the subtype of the array component with regard to size and alignment. In other words, the value specified must be at least equal to the size of this subtype, and must be a multiple of the alignment value.

In addition, component size clauses are allowed which cause the array to be packed, by specifying a smaller value.  The cases in which this is allowed are for component size values in the range 1 through 63.  The value specified must not be smaller than the Size of the subtype.  GNAT will accurately honor all packing requests in this range.  For example, if we have:

     type r is array (1 .. 8) of Natural;
     for r'Component_Size use 31;

then the resulting array has a length of 31 bytes (248 bits = 8 * 31). Of course access to the components of such an array is considerably less efficient than if the natural component size of 32 is used.

Bit_Order Clauses
=================

For record subtypes, GNAT permits the specification of the Bit_Order attribute.  The specification may either correspond to the default bit order for the target, in which case the specification has no effect and places no additional restrictions, or it may be for the non-standard setting (that is the opposite of the default).

In the case where the non-standard value is specified, the effect is to renumber bits within each byte, but the ordering of bytes is not affected.  There are certain restrictions placed on component clauses as follows:

  * Components fitting within a single storage unit.

     These are unrestricted, and the effect is merely to renumber bits.  For example if we are on a little-endian machine with Low_Order_First being the default, then the following two declarations have exactly the same effect:

             type R1 is record
                A : Boolean;
                B : Integer range 1 .. 120;
             end record;

             for R1 use record
                A at 0 range 0 .. 0;
                B at 0 range 1 .. 7;
             end record;

             type R2 is record
                A : Boolean;
                B : Integer range 1 .. 120;
             end record;

             for R2'Bit_Order use High_Order_First;

             for R2 use record
                A at 0 range 7 .. 7;
                B at 0 range 0 .. 6;
             end record;

     The useful application here is to write the second declaration with the Bit_Order attribute definition clause, and know that it will be treated the same, regardless of whether the target is little-endian or big-endian.

  * Components occupying an integral number of bytes.

     These are components that exactly fit in two or more bytes.  Such component declarations are allowed, but have no effect, since it is important to realize that the Bit_Order specification does not affect the ordering of bytes. In particular, the following attempt at getting an endian-independent integer does not work:

             type R2 is record
                A : Integer;
             end record;

             for R2'Bit_Order use High_Order_First;

             for R2 use record
                A at 0 range 0 .. 31;
             end record;

     This declaration will result in a little-endian integer on a little-endian machine, and a big-endian integer on a big-endian machine. If byte flipping is required for interoperability between big- and little-endian machines, this must be explicitly programmed.  This capability is not provided by Bit_Order.

  * Components that are positioned across byte boundaries

     but do not occupy an integral number of bytes.  Given that bytes are not reordered, such fields would occupy a non-contiguous sequence of bits in memory, requiring non-trivial code to reassemble.  They are for this reason not permitted, and any component clause specifying such a layout will be flagged as illegal by GNAT.

Since the misconception that Bit_Order automatically deals with all endian-related incompatibilities is a common one, the specification of a component field that is an integral number of bytes will always generate a warning.  This warning may be suppressed using pragma Suppress if desired.  The following section contains additional details regarding the issue of byte ordering.

Effect of Bit_Order on Byte Ordering
====================================

In this section we will review the effect of the Bit_Order attribute definition clause on byte ordering.  Briefly, it has no effect at all, but a detailed example will be helpful.  Before giving this example, let us review the precise definition of the effect of defining Bit_Order.  The effect of a non-standard bit order is described in section 15.5.3 of the Ada Reference Manual:

     2   A bit ordering is a method of interpreting the meaning of the storage place attributes.

To understand the precise definition of storage place attributes in this context, we visit section 13.5.1 of the manual:

     13   A record_representation_clause (without the mod_clause) specifies the layout.  The storage place attributes (see 13.5.2) are taken from the values of the position, first_bit, and last_bit expressions after normalizing those values so that first_bit is less than Storage_Unit.

The critical point here is that storage places are taken from the values after normalization, not before.  So the Bit_Order interpretation applies to normalized values.  The interpretation is described in the later part of the 15.5.3 paragraph:

     2   A bit ordering is a method of interpreting the meaning of the storage place attributes.  High_Order_First (known in the vernacular as ``big endian'') means that the first bit of a storage element (bit 0) is the most significant bit (interpreting the sequence of bits that represent a component as an unsigned integer value).  Low_Order_First (known in the vernacular as ``little endian'') means the opposite: the first bit is the least significant.

Note that the numbering is with respect to the bits of a storage unit.  In other words, the specification affects only the numbering of bits within a single storage unit.

We can make the effect clearer by giving an example.

Suppose that we have an external device which presents two bytes, the first byte presented, which is the first (low addressed byte) of the two byte record is called Master, and the second byte is called Slave.

The left most (most significant bit is called Control for each byte, and the remaining 7 bits are called V1, V2, ... V7, where V7 is the rightmost (least significant) bit.

On a big-endian machine, we can write the following representation clause

        type Data is record
           Master_Control : Bit;
           Master_V1      : Bit;
           Master_V2      : Bit;
           Master_V3      : Bit;
           Master_V4      : Bit;
           Master_V5      : Bit;
           Master_V6      : Bit;
           Master_V7      : Bit;
           Slave_Control  : Bit;
           Slave_V1       : Bit;
           Slave_V2       : Bit;
           Slave_V3       : Bit;
           Slave_V4       : Bit;
           Slave_V5       : Bit;
           Slave_V6       : Bit;
           Slave_V7       : Bit;
        end record;

        for Data use record
           Master_Control at 0 range 0 .. 0;
           Master_V1      at 0 range 1 .. 1;
           Master_V2      at 0 range 2 .. 2;
           Master_V3      at 0 range 3 .. 3;
           Master_V4      at 0 range 4 .. 4;
           Master_V5      at 0 range 5 .. 5;
           Master_V6      at 0 range 6 .. 6;
           Master_V7      at 0 range 7 .. 7;
           Slave_Control  at 1 range 0 .. 0;
           Slave_V1       at 1 range 1 .. 1;
           Slave_V2       at 1 range 2 .. 2;
           Slave_V3       at 1 range 3 .. 3;
           Slave_V4       at 1 range 4 .. 4;
           Slave_V5       at 1 range 5 .. 5;
           Slave_V6       at 1 range 6 .. 6;
           Slave_V7       at 1 range 7 .. 7;
        end record;

Now if we move this to a little endian machine, then the bit ordering within the byte is backwards, so we have to rewrite the record rep clause as:

        for Data use record
           Master_Control at 0 range 7 .. 7;
           Master_V1      at 0 range 6 .. 6;
           Master_V2      at 0 range 5 .. 5;
           Master_V3      at 0 range 4 .. 4;
           Master_V4      at 0 range 3 .. 3;
           Master_V5      at 0 range 2 .. 2;
           Master_V6      at 0 range 1 .. 1;
           Master_V7      at 0 range 0 .. 0;
           Slave_Control  at 1 range 7 .. 7;
           Slave_V1       at 1 range 6 .. 6;
           Slave_V2       at 1 range 5 .. 5;
           Slave_V3       at 1 range 4 .. 4;
           Slave_V4       at 1 range 3 .. 3;
           Slave_V5       at 1 range 2 .. 2;
           Slave_V6       at 1 range 1 .. 1;
           Slave_V7       at 1 range 0 .. 0;
        end record;

It is a nuisance to have to rewrite the clause, especially if the code has to be maintained on both machines.  However, this is a case that we can handle with the Bit_Order attribute if it is implemented. Note that the implementation is not required on byte addressed machines, but it is indeed implemented in GNAT. This means that we can simply use the first record clause, together with the declaration

        for Data'Bit_Order use High_Order_First;

and the effect is what is desired, namely the layout is exactly the same, independent of whether the code is compiled on a big-endian or little-endian machine.

The important point to understand is that byte ordering is not affected. A Bit_Order attribute definition never affects which byte a field ends up in, only where it ends up in that byte. To make this clear, let us rewrite the record rep clause of the previous example as:

        for Data'Bit_Order use High_Order_First;
        for Data use record
           Master_Control at 0 range  0 .. 0;
           Master_V1      at 0 range  1 .. 1;
           Master_V2      at 0 range  2 .. 2;
           Master_V3      at 0 range  3 .. 3;
           Master_V4      at 0 range  4 .. 4;
           Master_V5      at 0 range  5 .. 5;
           Master_V6      at 0 range  6 .. 6;
           Master_V7      at 0 range  7 .. 7;
           Slave_Control  at 0 range  8 .. 8;
           Slave_V1       at 0 range  9 .. 9;
           Slave_V2       at 0 range 10 .. 10;
           Slave_V3       at 0 range 11 .. 11;
           Slave_V4       at 0 range 12 .. 12;
           Slave_V5       at 0 range 13 .. 13;
           Slave_V6       at 0 range 14 .. 14;
           Slave_V7       at 0 range 15 .. 15;
        end record;

This is exactly equivalent to saying (a repeat of the first example):

        for Data'Bit_Order use High_Order_First;
        for Data use record
           Master_Control at 0 range 0 .. 0;
           Master_V1      at 0 range 1 .. 1;
           Master_V2      at 0 range 2 .. 2;
           Master_V3      at 0 range 3 .. 3;
           Master_V4      at 0 range 4 .. 4;
           Master_V5      at 0 range 5 .. 5;
           Master_V6      at 0 range 6 .. 6;
           Master_V7      at 0 range 7 .. 7;
           Slave_Control  at 1 range 0 .. 0;
           Slave_V1       at 1 range 1 .. 1;
           Slave_V2       at 1 range 2 .. 2;
           Slave_V3       at 1 range 3 .. 3;
           Slave_V4       at 1 range 4 .. 4;
           Slave_V5       at 1 range 5 .. 5;
           Slave_V6       at 1 range 6 .. 6;
           Slave_V7       at 1 range 7 .. 7;
        end record;

Why are they equivalent? Well take a specific field, the Slave_V2 field.  The storage place attributes are obtained by normalizing the values given so that the First_Bit value is less than 8.  After normalizing the values (0,10,10) we get (1,2,2) which is exactly what we specified in the other case.

Now one might expect that the Bit_Order attribute might affect bit numbering within the entire record component (two bytes in this case, thus affecting which byte fields end up in), but that is not the way this feature is defined, it only affects numbering of bits, not which byte they end up in.

Consequently it never makes sense to specify a starting bit number greater than 7 (for a byte addressable field) if an attribute definition for Bit_Order has been given, and indeed it may be actively confusing to specify such a value, so the compiler generates a warning for such usage.

If you do need to control byte ordering then appropriate conditional values must be used.  If in our example, the slave byte came first on some machines we might write:

        Master_Byte_First constant Boolean := ...;

        Master_Byte : constant Natural :=
                        1 - Boolean'Pos (Master_Byte_First);
        Slave_Byte  : constant Natural :=
                        Boolean'Pos (Master_Byte_First);

        for Data'Bit_Order use High_Order_First;
        for Data use record
           Master_Control at Master_Byte range 0 .. 0;
           Master_V1      at Master_Byte range 1 .. 1;
           Master_V2      at Master_Byte range 2 .. 2;
           Master_V3      at Master_Byte range 3 .. 3;
           Master_V4      at Master_Byte range 4 .. 4;
           Master_V5      at Master_Byte range 5 .. 5;
           Master_V6      at Master_Byte range 6 .. 6;
           Master_V7      at Master_Byte range 7 .. 7;
           Slave_Control  at Slave_Byte  range 0 .. 0;
           Slave_V1       at Slave_Byte  range 1 .. 1;
           Slave_V2       at Slave_Byte  range 2 .. 2;
           Slave_V3       at Slave_Byte  range 3 .. 3;
           Slave_V4       at Slave_Byte  range 4 .. 4;
           Slave_V5       at Slave_Byte  range 5 .. 5;
           Slave_V6       at Slave_Byte  range 6 .. 6;
           Slave_V7       at Slave_Byte  range 7 .. 7;
        end record;

Now to switch between machines, all that is necessary is to set the boolean constant Master_Byte_First in an appropriate manner.

Pragma Pack for Arrays
======================

Pragma Pack applied to an array has no effect unless the component type is packable.  For a component type to be packable, it must be one of the following cases:

  * Any scalar type

  * Any type whose size is specified with a size clause

  * Any packed array type with a static size

For all these cases, if the component subtype size is in the range 1 through 63, then the effect of the pragma Pack is exactly as though a component size were specified giving the component subtype size. For example if we have:

        type r is range 0 .. 17;

        type ar is array (1 .. 8) of r;
        pragma Pack (ar);

Then the component size of ar will be set to 5 (i.e. to r'size, and the size of the array ar will be exactly 40 bits.

Note that in some cases this rather fierce approach to packing can produce unexpected effects.  For example, in Ada 95, type Natural typically has a size of 31, meaning that if you pack an array of Natural, you get 31-bit close packing, which saves a few bits, but results in far less efficient access.  Since many other Ada compilers will ignore such a packing request, GNAT will generate a warning on some uses of pragma Pack that it guesses might not be what is intended.  You can easily remove this warning by using an explicit Component_Size setting instead, which never generates a warning, since the intention of the programmer is clear in this case.

GNAT treats packed arrays in one of two ways.  If the size of the array is known at compile time and is less than 64 bits, then internally the array is represented as a single modular type, of exactly the appropriate number of bits.  If the length is greater than 63 bits, or is not known at compile time, then the packed array is represented as an array of bytes, and the length is always a multiple of 8 bits.

Note that to represent a packed array as a modular type, the alignment must be suitable for the modular type involved. For example, on typical machines a 32-bit packed array will be represented by a 32-bit modular integer with an alignment of four bytes. If you explicitly override the default alignment with an alignment clause that is too small, the modular representation cannot be used. For example, consider the following set of declarations:

        type R is range 1 .. 3;
        type S is array (1 .. 31) of R;
        for S'Component_Size use 2;
        for S'Size use 62;
        for S'Alignment use 1;

If the alignment clause were not present, then a 62-bit modular representation would be chosen (typically with an alignment of 4 or 8 bytes depending on the target). But the default alignment is overridden with the explicit alignment clause. This means that the modular representation cannot be used, and instead the array of bytes representation must be used, meaning that the length must be a multiple of 8. Thus the above set of declarations will result in a diagnostic rejecting the size clause and noting that the minimum size allowed is 64.

One special case that is worth noting occurs when the base type of the component size is 8/16/32 and the subtype is one bit less. Notably this occurs with subtype Natural. Consider:

        type Arr is array (1 .. 32) of Natural;
        pragma Pack (Arr);

In all commonly used Ada 83 compilers, this pragma Pack would be ignored, since typically Natural'Size is 32 in Ada 83, and in any case most Ada 83 compilers did not attempt 31 bit packing.

In Ada 95, Natural'Size is required to be 31. Furthermore, GNAT really does pack 31-bit subtype to 31 bits. This may result in a substantial unintended performance penalty when porting legacy Ada 83 code. To help prevent this, GNAT generates a warning in such cases. If you really want 31 bit packing in a case like this, you can set the component size explicitly:

        type Arr is array (1 .. 32) of Natural;
        for Arr'Component_Size use 31;

Here 31-bit packing is achieved as required, and no warning is generated, since in this case the programmer intention is clear.

Pragma Pack for Records
=======================

Pragma Pack applied to a record will pack the components to reduce wasted space from alignment gaps and by reducing the amount of space taken by components.  We distinguish between packable components and non-packable components. Components of the following types are considered packable:

  * All primitive types are packable.

  * Small packed arrays, whose size does not exceed 64 bits, and where the size is statically known at compile time, are represented internally as modular integers, and so they are also packable.

All packable components occupy the exact number of bits corresponding to their Size value, and are packed with no padding bits, i.e. they can start on an arbitrary bit boundary.

All other types are non-packable, they occupy an integral number of storage units, and are placed at a boundary corresponding to their alignment requirements.

For example, consider the record

        type Rb1 is array (1 .. 13) of Boolean;
        pragma Pack (rb1);

        type Rb2 is array (1 .. 65) of Boolean;
        pragma Pack (rb2);

        type x2 is record
           l1 : Boolean;
           l2 : Duration;
           l3 : Float;
           l4 : Boolean;
           l5 : Rb1;
           l6 : Rb2;
        end record;
        pragma Pack (x2);

The representation for the record x2 is as follows:

     for x2'Size use 224;
     for x2 use record
        l1 at  0 range  0 .. 0;
        l2 at  0 range  1 .. 64;
        l3 at 12 range  0 .. 31;
        l4 at 16 range  0 .. 0;
        l5 at 16 range  1 .. 13;
        l6 at 18 range  0 .. 71;
     end record;

Studying this example, we see that the packable fields l1 and l2 are of length equal to their sizes, and placed at specific bit boundaries (and not byte boundaries) to eliminate padding.  But l3 is of a non-packable float type, so it is on the next appropriate alignment boundary.

The next two fields are fully packable, so l4 and l5 are minimally packed with no gaps.  However, type Rb2 is a packed array that is longer than 64 bits, so it is itself non-packable.  Thus the l6 field is aligned to the next byte boundary, and takes an integral number of bytes, i.e. 72 bits.

Record Representation Clauses
=============================

Record representation clauses may be given for all record types, including types obtained by record extension.  Component clauses are allowed for any static component.  The restrictions on component clauses depend on the type of the component.

For all components of an elementary type, the only restriction on component clauses is that the size must be at least the 'Size value of the type (actually the Value_Size).  There are no restrictions due to alignment, and such components may freely cross storage boundaries.

Packed arrays with a size up to and including 64 bits are represented internally using a modular type with the appropriate number of bits, and thus the same lack of restriction applies.  For example, if you declare:

        type R is array (1 .. 49) of Boolean;
        pragma Pack (R);
        for R'Size use 49;

then a component clause for a component of type R may start on any specified bit boundary, and may specify a value of 49 bits or greater.

Packed bit arrays that are longer than 64 bits must always be placed on a storage unit (byte) boundary. Any component clause that does not meet this requirement will be rejected.

The rules for other types are different for GNAT 3 and GNAT 5 versions (based on GCC 2 and GCC 3 respectively). In GNAT 5, larger components (other than packed arrays) may also be placed on arbitrary boundaries, so for example, the following is permitted:

        type R is array (1 .. 10) of Boolean;
        for R'Size use 80;

        type Q is record
           G, H : Boolean;
           L, M : R;
        end record;

        for Q use record
           G at 0 range  0 ..   0;
           H at 0 range  1 ..   1;
           L at 0 range  2 ..  81;
           R at 0 range 82 .. 161;
        end record;

In GNAT 3, there are more severe restrictions on larger components. For non-primitive types, including packed arrays with a size greater than 64 bits, component clauses must respect the alignment requirement of the type, in particular, always starting on a byte boundary, and the length must be a multiple of the storage unit.

The following rules regarding tagged types are enforced in both GNAT 3 and GNAT 5:

The tag field of a tagged type always occupies an address sized field at the start of the record.  No component clause may attempt to overlay this tag.

In the case of a record extension T1, of a type T, no component clause applied to the type T1 can specify a storage location that would overlap the first T'Size bytes of the record.

Enumeration Clauses
===================

The only restriction on enumeration clauses is that the range of values must be representable.  For the signed case, if one or more of the representation values are negative, all values must be in the range:

        System.Min_Int .. System.Max_Int

For the unsigned case, where all values are non negative, the values must be in the range:

        0 .. System.Max_Binary_Modulus;

A confirming representation clause is one in which the values range from 0 in sequence, i.e. a clause that confirms the default representation for an enumeration type. Such a confirming representation is permitted by these rules, and is specially recognized by the compiler so that no extra overhead results from the use of such a clause.

If an array has an index type which is an enumeration type to which an enumeration clause has been applied, then the array is stored in a compact manner.  Consider the declarations:

        type r is (A, B, C);
        for r use (A => 1, B => 5, C => 10);
        type t is array (r) of Character;

The array type t corresponds to a vector with exactly three elements and has a default size equal to 3*Character'Size.  This ensures efficient use of space, but means that accesses to elements of the array will incur the overhead of converting representation values to the corresponding positional values, (i.e. the value delivered by the Pos attribute).

Address Clauses
===============

The reference manual allows a general restriction on representation clauses, as found in RM 13.1(22):

     An implementation need not support representation items containing nonstatic expressions, except that an implementation should support a representation item for a given entity if each nonstatic expression in the representation item is a name that statically denotes a constant declared before the entity.

In practice this is applicable only to address clauses, since this is the only case in which a non-static expression is permitted by the syntax.  As the AARM notes in sections 13.1 (22.a-22.h):

       22.a   Reason: This is to avoid the following sort of thing:

       22.b        X : Integer := F(...);
                   Y : Address := G(...);
                   for X'Address use Y;

       22.c   In the above, we have to evaluate the
              initialization expression for X before we
              know where to put the result.  This seems
              like an unreasonable implementation burden.

       22.d   The above code should instead be written
              like this:

       22.e        Y : constant Address := G(...);
                   X : Integer := F(...);
                   for X'Address use Y;

       22.f   This allows the expression ``Y'' to be safely
              evaluated before X is created.

       22.g   The constant could be a formal parameter of mode in.

       22.h   An implementation can support other nonstatic
              expressions if it wants to.  Expressions of type
              Address are hardly ever static, but their value
              might be known at compile time anyway in many
              cases.

GNAT does indeed permit many additional cases of non-static expressions.  In particular, if the type involved is elementary there are no restrictions (since in this case, holding a temporary copy of the initialization value, if one is present, is inexpensive).  In addition, if there is no implicit or explicit initialization, then there are no restrictions.  GNAT will reject only the case where all three of these conditions hold:

  * The type of the item is non-elementary (e.g. a record or array).

  * There is explicit or implicit initialization required for the object. Note that access values are always implicitly initialized, and also in GNAT, certain bit-packed arrays (those having a dynamic length or a length greater than 64) will also be implicitly initialized to zero.

  * The address value is non-static.  Here GNAT is more permissive than the RM, and allows the address value to be the address of a previously declared stand-alone variable, as long as it does not itself have an address clause.

                     Anchor  : Some_Initialized_Type;
                     Overlay : Some_Initialized_Type;
                     for Overlay'Address use Anchor'Address;

     However, the prefix of the address clause cannot be an array component, or a component of a discriminated record.

As noted above in section 22.h, address values are typically non-static.  In particular the To_Address function, even if applied to a literal value, is a non-static function call.  To avoid this minor annoyance, GNAT provides the implementation defined attribute 'To_Address.  The following two expressions have identical values:

        To_Address (16#1234_0000#)
        System'To_Address (16#1234_0000#);

except that the second form is considered to be a static expression, and thus when used as an address clause value is always permitted.

Additionally, GNAT treats as static an address clause that is an unchecked_conversion of a static integer value.  This simplifies the porting of legacy code, and provides a portable equivalent to the GNAT attribute To_Address.

Another issue with address clauses is the interaction with alignment requirements.  When an address clause is given for an object, the address value must be consistent with the alignment of the object (which is usually the same as the alignment of the type of the object).  If an address clause is given that specifies an inappropriately aligned address value, then the program execution is erroneous.

Since this source of erroneous behavior can have unfortunate effects, GNAT checks (at compile time if possible, generating a warning, or at execution time with a run-time check) that the alignment is appropriate.  If the run-time check fails, then Program_Error is raised.  This run-time check is suppressed if range checks are suppressed, or if pragma Restrictions (No_Elaboration_Code) is in effect.

An address clause cannot be given for an exported object.  More understandably the real restriction is that objects with an address clause cannot be exported.  This is because such variables are not defined by the Ada program, so there is no external object to export.

It is permissible to give an address clause and a pragma Import for the same object.  In this case, the variable is not really defined by the Ada program, so there is no external symbol to be linked.  The link name and the external name are ignored in this case.  The reason that we allow this combination is that it provides a useful idiom to avoid unwanted initializations on objects with address clauses.

When an address clause is given for an object that has implicit or explicit initialization, then by default initialization takes place.  This means that the effect of the object declaration is to overwrite the memory at the specified address.  This is almost always not what the programmer wants, so GNAT will output a warning:

       with System;
       package G is
          type R is record
             M : Integer := 0;
          end record;

          Ext : R;
          for Ext'Address use System'To_Address (16#1234_1234#);
              |
       >>> warning: implicit initialization of "Ext" may
           modify overlaid storage
       >>> warning: use pragma Import for "Ext" to suppress
           initialization (RM B(24))

       end G;

As indicated by the warning message, the solution is to use a (dummy) pragma Import to suppress this initialization.  The pragma tell the compiler that the object is declared and initialized elsewhere.  The following package compiles without warnings (and the initialization is suppressed):

        with System;
        package G is
           type R is record
              M : Integer := 0;
           end record;

           Ext : R;
           for Ext'Address use System'To_Address (16#1234_1234#);
           pragma Import (Ada, Ext);
        end G;

A final issue with address clauses involves their use for overlaying variables, as in the following example:

       A : Integer;
       B : Integer;
       for B'Address use A'Address;

or alternatively, using the form recommended by the RM:

       A    : Integer;
       Addr : constant Address := A'Address;
       B    : Integer;
       for B'Address use Addr;

In both of these cases, A and B become aliased to one another via the address clause. This use of address clauses to overlay variables, achieving an effect similar to unchecked conversion was erroneous in Ada 83, but in Ada 95 the effect is implementation defined. Furthermore, the Ada 95 RM specifically recommends that in a situation like this, B should be subject to the following implementation advice (RM 13.3(19)):

     19  If the Address of an object is specified, or it is imported or exported, then the implementation should not perform optimizations based on assumptions of no aliases.

GNAT follows this recommendation, and goes further by also applying this recommendation to the overlaid variable (A in the above example) in this case. This means that the overlay works "as expected", in that a modification to one of the variables will affect the value of the other.

Effect of Convention on Representation
======================================

Normally the specification of a foreign language convention for a type or an object has no effect on the chosen representation.  In particular, the representation chosen for data in GNAT generally meets the standard system conventions, and for example records are laid out in a manner that is consistent with C.  This means that specifying convention C (for example) has no effect.

There are three exceptions to this general rule:

  * Convention Fortran and array subtypes If pragma Convention Fortran is specified for an array subtype, then in accordance with the implementation advice in section 3.6.2(11) of the Ada Reference Manual, the array will be stored in a Fortran-compatible column-major manner, instead of the normal default row-major order.

  * Convention C and enumeration types GNAT normally stores enumeration types in 8, 16, or 32 bits as required to accommodate all values of the type.  For example, for the enumeration type declared by:

             type Color is (Red, Green, Blue);

     8 bits is sufficient to store all values of the type, so by default, objects of type Color will be represented using 8 bits.  However, normal C convention is to use 32 bits for all enum values in C, since enum values are essentially of type int.  If pragma Convention C is specified for an Ada enumeration type, then the size is modified as necessary (usually to 32 bits) to be consistent with the C convention for enum values.

  * Convention C/Fortran and Boolean types In C, the usual convention for boolean values, that is values used for conditions, is that zero represents false, and nonzero values represent true.  In Ada, the normal convention is that two specific values, typically 0/1, are used to represent false/true respectively.

     Fortran has a similar convention for LOGICAL values (any nonzero value represents true).

     To accommodate the Fortran and C conventions, if a pragma Convention specifies C or Fortran convention for a derived Boolean, as in the following example:

             type C_Switch is new Boolean;
             pragma Convention (C, C_Switch);

     then the GNAT generated code will treat any nonzero value as true.  For truth values generated by GNAT, the conventional value 1 will be used for True, but when one of these values is read, any nonzero value is treated as True.

Determining the Representations chosen by GNAT
==============================================

Although the descriptions in this section are intended to be complete, it is often easier to simply experiment to see what GNAT accepts and what the effect is on the layout of types and objects.

As required by the Ada RM, if a representation clause is not accepted, then it must be rejected as illegal by the compiler.  However, when a representation clause or pragma is accepted, there can still be questions of what the compiler actually does.  For example, if a partial record representation clause specifies the location of some components and not others, then where are the non-specified components placed? Or if pragma Pack is used on a record, then exactly where are the resulting fields placed? The section on pragma Pack in this chapter can be used to answer the second question, but it is often easier to just see what the compiler does.

For this purpose, GNAT provides the option -gnatR.  If you compile with this option, then the compiler will output information on the actual representations chosen, in a format similar to source representation clauses.  For example, if we compile the package:

     package q is
        type r (x : boolean) is tagged record
           case x is
              when True => S : String (1 .. 100);
              when False => null;
           end case;
        end record;

        type r2 is new r (false) with record
           y2 : integer;
        end record;

        for r2 use record
           y2 at 16 range 0 .. 31;
        end record;

        type x is record
           y : character;
        end record;

        type x1 is array (1 .. 10) of x;
        for x1'component_size use 11;

        type ia is access integer;

        type Rb1 is array (1 .. 13) of Boolean;
        pragma Pack (rb1);

        type Rb2 is array (1 .. 65) of Boolean;
        pragma Pack (rb2);

        type x2 is record
           l1 : Boolean;
           l2 : Duration;
           l3 : Float;
           l4 : Boolean;
           l5 : Rb1;
           l6 : Rb2;
        end record;
        pragma Pack (x2);
     end q;

using the switch -gnatR we obtain the following output:

     Representation information for unit q
     -------------------------------------

     for r'Size use ??;
     for r'Alignment use 4;
     for r use record
        x    at 4 range  0 .. 7;
        _tag at 0 range  0 .. 31;
        s    at 5 range  0 .. 799;
     end record;

     for r2'Size use 160;
     for r2'Alignment use 4;
     for r2 use record
        x       at  4 range  0 .. 7;
        _tag    at  0 range  0 .. 31;
        _parent at  0 range  0 .. 63;
        y2      at 16 range  0 .. 31;
     end record;

     for x'Size use 8;
     for x'Alignment use 1;
     for x use record
        y at 0 range  0 .. 7;
     end record;

     for x1'Size use 112;
     for x1'Alignment use 1;
     for x1'Component_Size use 11;

     for rb1'Size use 13;
     for rb1'Alignment use 2;
     for rb1'Component_Size use 1;

     for rb2'Size use 72;
     for rb2'Alignment use 1;
     for rb2'Component_Size use 1;

     for x2'Size use 224;
     for x2'Alignment use 4;
     for x2 use record
        l1 at  0 range  0 .. 0;
        l2 at  0 range  1 .. 64;
        l3 at 12 range  0 .. 31;
        l4 at 16 range  0 .. 0;
        l5 at 16 range  1 .. 13;
        l6 at 18 range  0 .. 71;
     end record;

The Size values are actually the Object_Size, i.e. the default size that will be allocated for objects of the type. The ?? size for type r indicates that we have a variant record, and the actual size of objects will depend on the discriminant value.

The Alignment values show the actual alignment chosen by the compiler for each record or array type.

The record representation clause for type r shows where all fields are placed, including the compiler generated tag field (whose location cannot be controlled by the programmer).

The record representation clause for the type extension r2 shows all the fields present, including the parent field, which is a copy of the fields of the parent type of r2, i.e. r1.

The component size and size clauses for types rb1 and rb2 show the exact effect of pragma Pack on these arrays, and the record representation clause for type x2 shows how pragma Pack affects this record type.

In some cases, it may be useful to cut and paste the representation clauses generated by the compiler into the original source to fix and guarantee the actual representation to be used.

Standard Library Routines
*************************

The Ada 95 Reference Manual contains in Annex A a full description of an extensive set of standard library routines that can be used in any Ada program, and which must be provided by all Ada compilers.  They are analogous to the standard C library used by C programs.

GNAT implements all of the facilities described in annex A, and for most purposes the description in the Ada 95 reference manual, or appropriate Ada text book, will be sufficient for making use of these facilities.

In the case of the input-output facilities, see The Implementation of Standard I/O, gives details on exactly how GNAT interfaces to the file system.  For the remaining packages, the Ada 95 reference manual should be sufficient.  The following is a list of the packages included, together with a brief description of the functionality that is provided.

For completeness, references are included to other predefined library routines defined in other sections of the Ada 95 reference manual (these are cross-indexed from annex A).

Ada (A.2)
     This is a parent package for all the standard library packages.  It is usually included implicitly in your program, and itself contains no useful data or routines.

Ada.Calendar (9.6)
     Calendar provides time of day access, and routines for manipulating times and durations.

Ada.Characters (A.3.1)
     This is a dummy parent package that contains no useful entities

Ada.Characters.Handling (A.3.2)
     This package provides some basic character handling capabilities, including classification functions for classes of characters (e.g. test for letters, or digits).

Ada.Characters.Latin_1 (A.3.3)
     This package includes a complete set of definitions of the characters that appear in type CHARACTER.  It is useful for writing programs that will run in international environments.  For example, if you want an upper case E with an acute accent in a string, it is often better to use the definition of UC_E_Acute in this package.  Then your program will print in an understandable manner even if your environment does not support these extended characters.

Ada.Command_Line (A.15)
     This package provides access to the command line parameters and the name of the current program (analogous to the use of argc and argv in C), and also allows the exit status for the program to be set in a system-independent manner.

Ada.Decimal (F.2)
     This package provides constants describing the range of decimal numbers implemented, and also a decimal divide routine (analogous to the COBOL verb DIVIDE .. GIVING .. REMAINDER ..)

Ada.Direct_IO (A.8.4)
     This package provides input-output using a model of a set of records of fixed-length, containing an arbitrary definite Ada type, indexed by an integer record number.

Ada.Dynamic_Priorities (D.5)
     This package allows the priorities of a task to be adjusted dynamically as the task is running.

Ada.Exceptions (11.4.1)
     This package provides additional information on exceptions, and also contains facilities for treating exceptions as data objects, and raising exceptions with associated messages.

Ada.Finalization (7.6)
     This package contains the declarations and subprograms to support the use of controlled types, providing for automatic initialization and finalization (analogous to the constructors and destructors of C++)

Ada.Interrupts (C.3.2)
     This package provides facilities for interfacing to interrupts, which includes the set of signals or conditions that can be raised and recognized as interrupts.

Ada.Interrupts.Names (C.3.2)
     This package provides the set of interrupt names (actually signal or condition names) that can be handled by GNAT.

Ada.IO_Exceptions (A.13)
     This package defines the set of exceptions that can be raised by use of the standard IO packages.

Ada.Numerics
     This package contains some standard constants and exceptions used throughout the numerics packages.  Note that the constants pi and e are defined here, and it is better to use these definitions than rolling your own.

Ada.Numerics.Complex_Elementary_Functions
     Provides the implementation of standard elementary functions (such as log and trigonometric functions) operating on complex numbers using the standard Float and the Complex and Imaginary types created by the package Numerics.Complex_Types.

Ada.Numerics.Complex_Types
     This is a predefined instantiation of Numerics.Generic_Complex_Types using Standard.Float to build the type Complex and Imaginary.

Ada.Numerics.Discrete_Random
     This package provides a random number generator suitable for generating random integer values from a specified range.

Ada.Numerics.Float_Random
     This package provides a random number generator suitable for generating uniformly distributed floating point values.

Ada.Numerics.Generic_Complex_Elementary_Functions
     This is a generic version of the package that provides the implementation of standard elementary functions (such as log and trigonometric functions) for an arbitrary complex type.

     The following predefined instantiations of this package are provided:

     Short_Float
          Ada.Numerics.Short_Complex_Elementary_Functions

     Float
          Ada.Numerics.Complex_Elementary_Functions

     Long_Float
          Ada.Numerics. Long_Complex_Elementary_Functions

Ada.Numerics.Generic_Complex_Types
     This is a generic package that allows the creation of complex types, with associated complex arithmetic operations.

     The following predefined instantiations of this package exist

     Short_Float
          Ada.Numerics.Short_Complex_Complex_Types

     Float
          Ada.Numerics.Complex_Complex_Types

     Long_Float
          Ada.Numerics.Long_Complex_Complex_Types

Ada.Numerics.Generic_Elementary_Functions
     This is a generic package that provides the implementation of standard elementary functions (such as log an trigonometric functions) for an arbitrary float type.

     The following predefined instantiations of this package exist

     Short_Float
          Ada.Numerics.Short_Elementary_Functions

     Float
          Ada.Numerics.Elementary_Functions

     Long_Float
          Ada.Numerics.Long_Elementary_Functions

Ada.Real_Time (D.8)
     This package provides facilities similar to those of Calendar, but operating with a finer clock suitable for real time control. Note that annex D requires that there be no backward clock jumps, and GNAT generally guarantees this behavior, but of course if the external clock on which the GNAT runtime depends is deliberately reset by some external event, then such a backward jump may occur.

Ada.Sequential_IO (A.8.1)
     This package provides input-output facilities for sequential files, which can contain a sequence of values of a single type, which can be any Ada type, including indefinite (unconstrained) types.

Ada.Storage_IO (A.9)
     This package provides a facility for mapping arbitrary Ada types to and from a storage buffer.  It is primarily intended for the creation of new IO packages.

Ada.Streams (13.13.1)
     This is a generic package that provides the basic support for the concept of streams as used by the stream attributes (Input, Output, Read and Write).

Ada.Streams.Stream_IO (A.12.1)
     This package is a specialization of the type Streams defined in package Streams together with a set of operations providing Stream_IO capability.  The Stream_IO model permits both random and sequential access to a file which can contain an arbitrary set of values of one or more Ada types.

Ada.Strings (A.4.1)
     This package provides some basic constants used by the string handling packages.

Ada.Strings.Bounded (A.4.4)
     This package provides facilities for handling variable length strings.  The bounded model requires a maximum length.  It is thus somewhat more limited than the unbounded model, but avoids the use of dynamic allocation or finalization.

Ada.Strings.Fixed (A.4.3)
     This package provides facilities for handling fixed length strings.

Ada.Strings.Maps (A.4.2)
     This package provides facilities for handling character mappings and arbitrarily defined subsets of characters.  For instance it is useful in defining specialized translation tables.

Ada.Strings.Maps.Constants (A.4.6)
     This package provides a standard set of predefined mappings and predefined character sets.  For example, the standard upper to lower case conversion table is found in this package.  Note that upper to lower case conversion is non-trivial if you want to take the entire set of characters, including extended characters like E with an acute accent, into account.  You should use the mappings in this package (rather than adding 32 yourself) to do case mappings.

Ada.Strings.Unbounded (A.4.5)
     This package provides facilities for handling variable length strings.  The unbounded model allows arbitrary length strings, but requires the use of dynamic allocation and finalization.

Ada.Strings.Wide_Bounded (A.4.7)
Ada.Strings.Wide_Fixed (A.4.7)
Ada.Strings.Wide_Maps (A.4.7)
Ada.Strings.Wide_Maps.Constants (A.4.7)
Ada.Strings.Wide_Unbounded (A.4.7)
     These packages provide analogous capabilities to the corresponding packages without Wide_ in the name, but operate with the types Wide_String and Wide_Character instead of String and Character.

Ada.Strings.Wide_Wide_Bounded (A.4.7)
Ada.Strings.Wide_Wide_Fixed (A.4.7)
Ada.Strings.Wide_Wide_Maps (A.4.7)
Ada.Strings.Wide_Wide_Maps.Constants (A.4.7)
Ada.Strings.Wide_Wide_Unbounded (A.4.7)
     These packages provide analogous capabilities to the corresponding packages without Wide_ in the name, but operate with the types Wide_Wide_String and Wide_Wide_Character instead of String and Character.

Ada.Synchronous_Task_Control (D.10)
     This package provides some standard facilities for controlling task communication in a synchronous manner.

Ada.Tags
     This package contains definitions for manipulation of the tags of tagged values.

Ada.Task_Attributes
     This package provides the capability of associating arbitrary task-specific data with separate tasks.

Ada.Text_IO
     This package provides basic text input-output capabilities for character, string and numeric data.  The subpackages of this package are listed next.

Ada.Text_IO.Decimal_IO
     Provides input-output facilities for decimal fixed-point types

Ada.Text_IO.Enumeration_IO
     Provides input-output facilities for enumeration types.

Ada.Text_IO.Fixed_IO
     Provides input-output facilities for ordinary fixed-point types.

Ada.Text_IO.Float_IO
     Provides input-output facilities for float types.  The following predefined instantiations of this generic package are available:

     Short_Float
          Short_Float_Text_IO

     Float
          Float_Text_IO

     Long_Float
          Long_Float_Text_IO

Ada.Text_IO.Integer_IO
     Provides input-output facilities for integer types.  The following predefined instantiations of this generic package are available:

     Short_Short_Integer
          Ada.Short_Short_Integer_Text_IO

     Short_Integer
          Ada.Short_Integer_Text_IO

     Integer
          Ada.Integer_Text_IO

     Long_Integer
          Ada.Long_Integer_Text_IO

     Long_Long_Integer
          Ada.Long_Long_Integer_Text_IO

Ada.Text_IO.Modular_IO
     Provides input-output facilities for modular (unsigned) types

Ada.Text_IO.Complex_IO (G.1.3)
     This package provides basic text input-output capabilities for complex data.

Ada.Text_IO.Editing (F.3.3)
     This package contains routines for edited output, analogous to the use of pictures in COBOL.  The picture formats used by this package are a close copy of the facility in COBOL.

Ada.Text_IO.Text_Streams (A.12.2)
     This package provides a facility that allows Text_IO files to be treated as streams, so that the stream attributes can be used for writing arbitrary data, including binary data, to Text_IO files.

Ada.Unchecked_Conversion (13.9)
     This generic package allows arbitrary conversion from one type to another of the same size, providing for breaking the type safety in special circumstances.

     If the types have the same Size (more accurately the same Value_Size), then the effect is simply to transfer the bits from the source to the target type without any modification.  This usage is well defined, and for simple types whose representation is typically the same across all implementations, gives a portable method of performing such conversions.

     If the types do not have the same size, then the result is implementation defined, and thus may be non-portable.  The following describes how GNAT handles such unchecked conversion cases.

     If the types are of different sizes, and are both discrete types, then the effect is of a normal type conversion without any constraint checking. In particular if the result type has a larger size, the result will be zero or sign extended.  If the result type has a smaller size, the result will be truncated by ignoring high order bits.

     If the types are of different sizes, and are not both discrete types, then the conversion works as though pointers were created to the source and target, and the pointer value is converted.  The effect is that bits are copied from successive low order storage units and bits of the source up to the length of the target type.

     A warning is issued if the lengths differ, since the effect in this case is implementation dependent, and the above behavior may not match that of some other compiler.

     A pointer to one type may be converted to a pointer to another type using unchecked conversion.  The only case in which the effect is undefined is when one or both pointers are pointers to unconstrained array types.  In this case, the bounds information may get incorrectly transferred, and in particular, GNAT uses double size pointers for such types, and it is meaningless to convert between such pointer types.  GNAT will issue a warning if the alignment of the target designated type is more strict than the alignment of the source designated type (since the result may be unaligned in this case).

     A pointer other than a pointer to an unconstrained array type may be converted to and from System.Address.  Such usage is common in Ada 83 programs, but note that Ada.Address_To_Access_Conversions is the preferred method of performing such conversions in Ada 95.  Neither unchecked conversion nor Ada.Address_To_Access_Conversions should be used in conjunction with pointers to unconstrained objects, since the bounds information cannot be handled correctly in this case.

Ada.Unchecked_Deallocation (13.11.2)
     This generic package allows explicit freeing of storage previously allocated by use of an allocator.

Ada.Wide_Text_IO (A.11)
     This package is similar to Ada.Text_IO, except that the external file supports wide character representations, and the internal types are Wide_Character and Wide_String instead of Character and String.  It contains generic subpackages listed next.

Ada.Wide_Text_IO.Decimal_IO
     Provides input-output facilities for decimal fixed-point types

Ada.Wide_Text_IO.Enumeration_IO
     Provides input-output facilities for enumeration types.

Ada.Wide_Text_IO.Fixed_IO
     Provides input-output facilities for ordinary fixed-point types.

Ada.Wide_Text_IO.Float_IO
     Provides input-output facilities for float types.  The following predefined instantiations of this generic package are available:

     Short_Float
          Short_Float_Wide_Text_IO

     Float
          Float_Wide_Text_IO

     Long_Float
          Long_Float_Wide_Text_IO

Ada.Wide_Text_IO.Integer_IO
     Provides input-output facilities for integer types.  The following predefined instantiations of this generic package are available:

     Short_Short_Integer
          Ada.Short_Short_Integer_Wide_Text_IO

     Short_Integer
          Ada.Short_Integer_Wide_Text_IO

     Integer
          Ada.Integer_Wide_Text_IO

     Long_Integer
          Ada.Long_Integer_Wide_Text_IO

     Long_Long_Integer
          Ada.Long_Long_Integer_Wide_Text_IO

Ada.Wide_Text_IO.Modular_IO
     Provides input-output facilities for modular (unsigned) types

Ada.Wide_Text_IO.Complex_IO (G.1.3)
     This package is similar to Ada.Text_IO.Complex_IO, except that the external file supports wide character representations.

Ada.Wide_Text_IO.Editing (F.3.4)
     This package is similar to Ada.Text_IO.Editing, except that the types are Wide_Character and Wide_String instead of Character and String.

Ada.Wide_Text_IO.Streams (A.12.3)
     This package is similar to Ada.Text_IO.Streams, except that the types are Wide_Character and Wide_String instead of Character and String.

Ada.Wide_Wide_Text_IO (A.11)
     This package is similar to Ada.Text_IO, except that the external file supports wide character representations, and the internal types are Wide_Character and Wide_String instead of Character and String.  It contains generic subpackages listed next.

Ada.Wide_Wide_Text_IO.Decimal_IO
     Provides input-output facilities for decimal fixed-point types

Ada.Wide_Wide_Text_IO.Enumeration_IO
     Provides input-output facilities for enumeration types.

Ada.Wide_Wide_Text_IO.Fixed_IO
     Provides input-output facilities for ordinary fixed-point types.

Ada.Wide_Wide_Text_IO.Float_IO
     Provides input-output facilities for float types.  The following predefined instantiations of this generic package are available:

     Short_Float
          Short_Float_Wide_Wide_Text_IO

     Float
          Float_Wide_Wide_Text_IO

     Long_Float
          Long_Float_Wide_Wide_Text_IO

Ada.Wide_Wide_Text_IO.Integer_IO
     Provides input-output facilities for integer types.  The following predefined instantiations of this generic package are available:

     Short_Short_Integer
          Ada.Short_Short_Integer_Wide_Wide_Text_IO

     Short_Integer
          Ada.Short_Integer_Wide_Wide_Text_IO

     Integer
          Ada.Integer_Wide_Wide_Text_IO

     Long_Integer
          Ada.Long_Integer_Wide_Wide_Text_IO

     Long_Long_Integer
          Ada.Long_Long_Integer_Wide_Wide_Text_IO

Ada.Wide_Wide_Text_IO.Modular_IO
     Provides input-output facilities for modular (unsigned) types

Ada.Wide_Wide_Text_IO.Complex_IO (G.1.3)
     This package is similar to Ada.Text_IO.Complex_IO, except that the external file supports wide character representations.

Ada.Wide_Wide_Text_IO.Editing (F.3.4)
     This package is similar to Ada.Text_IO.Editing, except that the types are Wide_Character and Wide_String instead of Character and String.

Ada.Wide_Wide_Text_IO.Streams (A.12.3)
     This package is similar to Ada.Text_IO.Streams, except that the types are Wide_Character and Wide_String instead of Character and String.

The Implementation of Standard I/O
**********************************

GNAT implements all the required input-output facilities described in A.6 through A.14.  These sections of the Ada 95 reference manual describe the required behavior of these packages from the Ada point of view, and if you are writing a portable Ada program that does not need to know the exact manner in which Ada maps to the outside world when it comes to reading or writing external files, then you do not need to read this chapter.  As long as your files are all regular files (not pipes or devices), and as long as you write and read the files only from Ada, the description in the Ada 95 reference manual is sufficient.

However, if you want to do input-output to pipes or other devices, such as the keyboard or screen, or if the files you are dealing with are either generated by some other language, or to be read by some other language, then you need to know more about the details of how the GNAT implementation of these input-output facilities behaves.

In this chapter we give a detailed description of exactly how GNAT interfaces to the file system.  As always, the sources of the system are available to you for answering questions at an even more detailed level, but for most purposes the information in this chapter will suffice.

Another reason that you may need to know more about how input-output is implemented arises when you have a program written in mixed languages where, for example, files are shared between the C and Ada sections of the same program.  GNAT provides some additional facilities, in the form of additional child library packages, that facilitate this sharing, and these additional facilities are also described in this chapter.

Standard I/O Packages
=====================

The Standard I/O packages described in Annex A for

  * Ada.Text_IO

  * Ada.Text_IO.Complex_IO

  * Ada.Text_IO.Text_Streams

  * Ada.Wide_Text_IO

  * Ada.Wide_Text_IO.Complex_IO

  * Ada.Wide_Text_IO.Text_Streams

  * Ada.Wide_Wide_Text_IO

  * Ada.Wide_Wide_Text_IO.Complex_IO

  * Ada.Wide_Wide_Text_IO.Text_Streams

  * Ada.Stream_IO

  * Ada.Sequential_IO

  * Ada.Direct_IO

are implemented using the C library streams facility; where

  * All files are opened using fopen.

  * All input/output operations use fread/fwrite.

There is no internal buffering of any kind at the Ada library level.  The only buffering is that provided at the system level in the implementation of the C library routines that support streams.  This facilitates shared use of these streams by mixed language programs.

FORM Strings
============

The format of a FORM string in GNAT is:

     "keyword=value,keyword=value,...,keyword=value"

where letters may be in upper or lower case, and there are no spaces between values.  The order of the entries is not important.  Currently there are two keywords defined.

     SHARED=[YES|NO]
     WCEM=[n|h|u|s\e]

The use of these parameters is described later in this section.

Direct_IO
=========

Direct_IO can only be instantiated for definite types.  This is a restriction of the Ada language, which means that the records are fixed length (the length being determined by TYPE'Size, rounded up to the next storage unit boundary if necessary).

The records of a Direct_IO file are simply written to the file in index sequence, with the first record starting at offset zero, and subsequent records following.  There is no control information of any kind.  For example, if 32-bit integers are being written, each record takes 4-bytes, so the record at index K starts at offset (K-1)*4.

There is no limit on the size of Direct_IO files, they are expanded as necessary to accommodate whatever records are written to the file.

Sequential_IO
=============

Sequential_IO may be instantiated with either a definite (constrained) or indefinite (unconstrained) type.

For the definite type case, the elements written to the file are simply the memory images of the data values with no control information of any kind.  The resulting file should be read using the same type, no validity checking is performed on input.

For the indefinite type case, the elements written consist of two parts.  First is the size of the data item, written as the memory image of a Interfaces.C.size_t value, followed by the memory image of the data value.  The resulting file can only be read using the same (unconstrained) type.  Normal assignment checks are performed on these read operations, and if these checks fail, Data_Error is raised.  In particular, in the array case, the lengths must match, and in the variant record case, if the variable for a particular read operation is constrained, the discriminants must match.

Note that it is not possible to use Sequential_IO to write variable length array items, and then read the data back into different length arrays.  For example, the following will raise Data_Error:

      package IO is new Sequential_IO (String);
      F : IO.File_Type;
      S : String (1..4);
      ...
      IO.Create (F)
      IO.Write (F, "hello!")
      IO.Reset (F, Mode=>In_File);
      IO.Read (F, S);
      Put_Line (S);

On some Ada implementations, this will print hell, but the program is clearly incorrect, since there is only one element in the file, and that element is the string hello!.

In Ada 95, this kind of behavior can be legitimately achieved using Stream_IO, and this is the preferred mechanism.  In particular, the above program fragment rewritten to use Stream_IO will work correctly.

Text_IO
=======

Text_IO files consist of a stream of characters containing the following special control characters:

     LF (line feed, 16#0A#) Line Mark
     FF (form feed, 16#0C#) Page Mark

A canonical Text_IO file is defined as one in which the following conditions are met:

  * The character LF is used only as a line mark, i.e. to mark the end of the line.

  * The character FF is used only as a page mark, i.e. to mark the end of a page and consequently can appear only immediately following a LF (line mark) character.

  * The file ends with either LF (line mark) or LF-FF (line mark, page mark).  In the former case, the page mark is implicitly assumed to be present.

A file written using Text_IO will be in canonical form provided that no explicit LF or FF characters are written using Put or Put_Line.  There will be no FF character at the end of the file unless an explicit New_Page operation was performed before closing the file.

A canonical Text_IO file that is a regular file, i.e. not a device or a pipe, can be read using any of the routines in Text_IO.  The semantics in this case will be exactly as defined in the Ada 95 reference manual and all the routines in Text_IO are fully implemented.

A text file that does not meet the requirements for a canonical Text_IO file has one of the following:

  * The file contains FF characters not immediately following a LF character.

  * The file contains LF or FF characters written by Put or Put_Line, which are not logically considered to be line marks or page marks.

  * The file ends in a character other than LF or FF, i.e. there is no explicit line mark or page mark at the end of the file.

Text_IO can be used to read such non-standard text files but subprograms to do with line or page numbers do not have defined meanings.  In particular, a FF character that does not follow a LF character may or may not be treated as a page mark from the point of view of page and line numbering.  Every LF character is considered to end a line, and there is an implied LF character at the end of the file.

Stream Pointer Positioning
--------------------------

Ada.Text_IO has a definition of current position for a file that is being read.  No internal buffering occurs in Text_IO, and usually the physical position in the stream used to implement the file corresponds to this logical position defined by Text_IO.  There are two exceptions:

  * After a call to End_Of_Page that returns True, the stream is positioned past the LF (line mark) that precedes the page mark.  Text_IO maintains an internal flag so that subsequent read operations properly handle the logical position which is unchanged by the End_Of_Page call.

  * After a call to End_Of_File that returns True, if the Text_IO file was positioned before the line mark at the end of file before the call, then the logical position is unchanged, but the stream is physically positioned right at the end of file (past the line mark, and past a possible page mark following the line mark.  Again Text_IO maintains internal flags so that subsequent read operations properly handle the logical position.

These discrepancies have no effect on the observable behavior of Text_IO, but if a single Ada stream is shared between a C program and Ada program, or shared (using shared=yes in the form string) between two Ada files, then the difference may be observable in some situations.

Reading and Writing Non-Regular Files
-------------------------------------

A non-regular file is a device (such as a keyboard), or a pipe.  Text_IO can be used for reading and writing.  Writing is not affected and the sequence of characters output is identical to the normal file case, but for reading, the behavior of Text_IO is modified to avoid undesirable look-ahead as follows:

An input file that is not a regular file is considered to have no page marks.  Any Ascii.FF characters (the character normally used for a page mark) appearing in the file are considered to be data characters.  In particular:

  * Get_Line and Skip_Line do not test for a page mark following a line mark.  If a page mark appears, it will be treated as a data character.

  * This avoids the need to wait for an extra character to be typed or entered from the pipe to complete one of these operations.

  * End_Of_Page always returns False

  * End_Of_File will return False if there is a page mark at the end of the file.

Output to non-regular files is the same as for regular files.  Page marks may be written to non-regular files using New_Page, but as noted above they will not be treated as page marks on input if the output is piped to another Ada program.

Another important discrepancy when reading non-regular files is that the end of file indication is not ``sticky''.  If an end of file is entered, e.g. by pressing the EOT key, then end of file is signaled once (i.e. the test End_Of_File will yield True, or a read will raise End_Error), but then reading can resume to read data past that end of file indication, until another end of file indication is entered.

Get_Immediate
-------------

Get_Immediate returns the next character (including control characters) from the input file.  In particular, Get_Immediate will return LF or FF characters used as line marks or page marks.  Such operations leave the file positioned past the control character, and it is thus not treated as having its normal function.  This means that page, line and column counts after this kind of Get_Immediate call are set as though the mark did not occur.  In the case where a Get_Immediate leaves the file positioned between the line mark and page mark (which is not normally possible), it is undefined whether the FF character will be treated as a page mark.

Treating Text_IO Files as Streams
---------------------------------

The package Text_IO.Streams allows a Text_IO file to be treated as a stream.  Data written to a Text_IO file in this stream mode is binary data.  If this binary data contains bytes 16#0A# (LF) or 16#0C# (FF), the resulting file may have non-standard format.  Similarly if read operations are used to read from a Text_IO file treated as a stream, then LF and FF characters may be skipped and the effect is similar to that described above for Get_Immediate.

Text_IO Extensions
------------------

A package GNAT.IO_Aux in the GNAT library provides some useful extensions to the standard Text_IO package:

  * function File_Exists (Name : String) return Boolean; Determines if a file of the given name exists.

  * function Get_Line return String; Reads a string from the standard input file.  The value returned is exactly the length of the line that was read.

  * function Get_Line (File : Ada.Text_IO.File_Type) return String; Similar, except that the parameter File specifies the file from which the string is to be read.

Text_IO Facilities for Unbounded Strings
----------------------------------------

The package Ada.Strings.Unbounded.Text_IO in library files a-suteio.ads/adb contains some GNAT-specific subprograms useful for Text_IO operations on unbounded strings:

  * function Get_Line (File : File_Type) return Unbounded_String; Reads a line from the specified file and returns the result as an unbounded string.

  * procedure Put (File : File_Type; U : Unbounded_String); Writes the value of the given unbounded string to the specified file Similar to the effect of Put (To_String (U)) except that an extra copy is avoided.

  * procedure Put_Line (File : File_Type; U : Unbounded_String); Writes the value of the given unbounded string to the specified file, followed by a New_Line. Similar to the effect of Put_Line (To_String (U)) except that an extra copy is avoided.

In the above procedures, File is of type Ada.Text_IO.File_Type and is optional.  If the parameter is omitted, then the standard input or output file is referenced as appropriate.

The package Ada.Strings.Wide_Unbounded.Wide_Text_IO in library files a-swuwti.ads and a-swuwti.adb provides similar extended Wide_Text_IO functionality for unbounded wide strings.

The package Ada.Strings.Wide_Wide_Unbounded.Wide_Wide_Text_IO in library files a-szuzti.ads and a-szuzti.adb provides similar extended Wide_Wide_Text_IO functionality for unbounded wide wide strings.

Wide_Text_IO
============

Wide_Text_IO is similar in most respects to Text_IO, except that both input and output files may contain special sequences that represent wide character values.  The encoding scheme for a given file may be specified using a FORM parameter:

     WCEM=X

as part of the FORM string (WCEM = wide character encoding method), where X is one of the following characters

h
     Hex ESC encoding

u
     Upper half encoding

s
     Shift-JIS encoding

e
     EUC Encoding

8
     UTF-8 encoding

b
     Brackets encoding

The encoding methods match those that can be used in a source program, but there is no requirement that the encoding method used for the source program be the same as the encoding method used for files, and different files may use different encoding methods.

The default encoding method for the standard files, and for opened files for which no WCEM parameter is given in the FORM string matches the wide character encoding specified for the main program (the default being brackets encoding if no coding method was specified with -gnatW).

Hex Coding
     In this encoding, a wide character is represented by a five character sequence:

          ESC a b c d

     where A, B, C, D are the four hexadecimal characters (using upper case letters) of the wide character code.  For example, ESC A345 is used to represent the wide character with code 16#A345#.  This scheme is compatible with use of the full Wide_Character set.

Upper Half Coding
     The wide character with encoding 16#abcd#, where the upper bit is on (i.e. a is in the range 8-F) is represented as two bytes 16#ab# and 16#cd#.  The second byte may never be a format control character, but is not required to be in the upper half.  This method can be also used for shift-JIS or EUC where the internal coding matches the external coding.

Shift JIS Coding
     A wide character is represented by a two character sequence 16#ab# and 16#cd#, with the restrictions described for upper half encoding as described above.  The internal character code is the corresponding JIS character according to the standard algorithm for Shift-JIS conversion.  Only characters defined in the JIS code set table can be used with this encoding method.

EUC Coding
     A wide character is represented by a two character sequence 16#ab# and 16#cd#, with both characters being in the upper half.  The internal character code is the corresponding JIS character according to the EUC encoding algorithm.  Only characters defined in the JIS code set table can be used with this encoding method.

UTF-8 Coding
     A wide character is represented using UCS Transformation Format 8 (UTF-8) as defined in Annex R of ISO 10646-1/Am.2.  Depending on the character value, the representation is a one, two, or three byte sequence:

          16#0000#-16#007f#: 2#0xxxxxxx#
          16#0080#-16#07ff#: 2#110xxxxx# 2#10xxxxxx#
          16#0800#-16#ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx#

     where the xxx bits correspond to the left-padded bits of the 16-bit character value.  Note that all lower half ASCII characters are represented as ASCII bytes and all upper half characters and other wide characters are represented as sequences of upper-half (The full UTF-8 scheme allows for encoding 31-bit characters as 6-byte sequences, but in this implementation, all UTF-8 sequences of four or more bytes length will raise a Constraint_Error, as will all invalid UTF-8 sequences.)

Brackets Coding
     In this encoding, a wide character is represented by the following eight character sequence:

          [ " a b c d " ]

     where a, b, c, d are the four hexadecimal characters (using uppercase letters) of the wide character code.  For example, ["A345"] is used to represent the wide character with code 16#A345#. This scheme is compatible with use of the full Wide_Character set. On input, brackets coding can also be used for upper half characters, e.g. ["C1"] for lower case a.  However, on output, brackets notation is only used for wide characters with a code greater than 16#FF#.

For the coding schemes other than Hex and Brackets encoding, not all wide character values can be represented.  An attempt to output a character that cannot be represented using the encoding scheme for the file causes Constraint_Error to be raised.  An invalid wide character sequence on input also causes Constraint_Error to be raised.

Stream Pointer Positioning
--------------------------

Ada.Wide_Text_IO is similar to Ada.Text_IO in its handling of stream pointer positioning (see Text_IO).  There is one additional case:

If Ada.Wide_Text_IO.Look_Ahead reads a character outside the normal lower ASCII set (i.e. a character in the range:

     Wide_Character'Val (16#0080#) .. Wide_Character'Val (16#FFFF#)

then although the logical position of the file pointer is unchanged by the Look_Ahead call, the stream is physically positioned past the wide character sequence.  Again this is to avoid the need for buffering or backup, and all Wide_Text_IO routines check the internal indication that this situation has occurred so that this is not visible to a normal program using Wide_Text_IO.  However, this discrepancy can be observed if the wide text file shares a stream with another file.

Reading and Writing Non-Regular Files
-------------------------------------

As in the case of Text_IO, when a non-regular file is read, it is assumed that the file contains no page marks (any form characters are treated as data characters), and End_Of_Page always returns False.  Similarly, the end of file indication is not sticky, so it is possible to read beyond an end of file.

Wide_Wide_Text_IO
=================

Wide_Wide_Text_IO is similar in most respects to Text_IO, except that both input and output files may contain special sequences that represent wide wide character values.  The encoding scheme for a given file may be specified using a FORM parameter:

     WCEM=X

as part of the FORM string (WCEM = wide character encoding method), where X is one of the following characters

h
     Hex ESC encoding

u
     Upper half encoding

s
     Shift-JIS encoding

e
     EUC Encoding

8
     UTF-8 encoding

b
     Brackets encoding

The encoding methods match those that can be used in a source program, but there is no requirement that the encoding method used for the source program be the same as the encoding method used for files, and different files may use different encoding methods.

The default encoding method for the standard files, and for opened files for which no WCEM parameter is given in the FORM string matches the wide character encoding specified for the main program (the default being brackets encoding if no coding method was specified with -gnatW).

UTF-8 Coding
     A wide character is represented using UCS Transformation Format 8 (UTF-8) as defined in Annex R of ISO 10646-1/Am.2.  Depending on the character value, the representation is a one, two, three, or four byte sequence:

          16#000000#-16#00007f#: 2#0xxxxxxx#
          16#000080#-16#0007ff#: 2#110xxxxx# 2#10xxxxxx#
          16#000800#-16#00ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx#
          16#010000#-16#10ffff#: 2#11110xxx# 2#10xxxxxx# 2#10xxxxxx# 2#10xxxxxx#

     where the xxx bits correspond to the left-padded bits of the 21-bit character value.  Note that all lower half ASCII characters are represented as ASCII bytes and all upper half characters and other wide characters are represented as sequences of upper-half characters.

Brackets Coding
     In this encoding, a wide wide character is represented by the following eight character sequence if is in wide character range

          [ " a b c d " ]

     and by the following ten character sequence if not

          [ " a b c d e f " ]

     where a, b, c, d, e, and f are the four or six hexadecimal characters (using uppercase letters) of the wide wide character code.  For example, ["01A345"] is used to represent the wide wide character with code 16#01A345#.

     This scheme is compatible with use of the full Wide_Wide_Character set. On input, brackets coding can also be used for upper half characters, e.g. ["C1"] for lower case a.  However, on output, brackets notation is only used for wide characters with a code greater than 16#FF#.

If is also possible to use the other Wide_Character encoding methods, such as Shift-JIS, but the other schemes cannot support the full range of wide wide characters. An attempt to output a character that cannot be represented using the encoding scheme for the file causes Constraint_Error to be raised.  An invalid wide character sequence on input also causes Constraint_Error to be raised.

Stream Pointer Positioning
--------------------------

Ada.Wide_Wide_Text_IO is similar to Ada.Text_IO in its handling of stream pointer positioning (see Text_IO).  There is one additional case:

If Ada.Wide_Wide_Text_IO.Look_Ahead reads a character outside the normal lower ASCII set (i.e. a character in the range:

     Wide_Wide_Character'Val (16#0080#) .. Wide_Wide_Character'Val (16#10FFFF#)

then although the logical position of the file pointer is unchanged by the Look_Ahead call, the stream is physically positioned past the wide character sequence.  Again this is to avoid the need for buffering or backup, and all Wide_Wide_Text_IO routines check the internal indication that this situation has occurred so that this is not visible to a normal program using Wide_Wide_Text_IO.  However, this discrepancy can be observed if the wide text file shares a stream with another file.

Reading and Writing Non-Regular Files
-------------------------------------

As in the case of Text_IO, when a non-regular file is read, it is assumed that the file contains no page marks (any form characters are treated as data characters), and End_Of_Page always returns False.  Similarly, the end of file indication is not sticky, so it is possible to read beyond an end of file.

Stream_IO
=========

A stream file is a sequence of bytes, where individual elements are written to the file as described in the Ada 95 reference manual.  The type Stream_Element is simply a byte.  There are two ways to read or write a stream file.

  * The operations Read and Write directly read or write a sequence of stream elements with no control information.

  * The stream attributes applied to a stream file transfer data in the manner described for stream attributes.

Shared Files
============

Section A.14 of the Ada 95 Reference Manual allows implementations to provide a wide variety of behavior if an attempt is made to access the same external file with two or more internal files.

To provide a full range of functionality, while at the same time minimizing the problems of portability caused by this implementation dependence, GNAT handles file sharing as follows:

  * In the absence of a shared=XXX form parameter, an attempt to open two or more files with the same full name is considered an error and is not supported.  The exception Use_Error will be raised.  Note that a file that is not explicitly closed by the program remains open until the program terminates.

  * If the form parameter shared=no appears in the form string, the file can be opened or created with its own separate stream identifier, regardless of whether other files sharing the same external file are opened.  The exact effect depends on how the C stream routines handle multiple accesses to the same external files using separate streams.

  * If the form parameter shared=yes appears in the form string for each of two or more files opened using the same full name, the same stream is shared between these files, and the semantics are as described in Ada 95 Reference Manual, Section A.14.

When a program that opens multiple files with the same name is ported from another Ada compiler to GNAT, the effect will be that Use_Error is raised.

The documentation of the original compiler and the documentation of the program should then be examined to determine if file sharing was expected, and shared=XXX parameters added to Open and Create calls as required.

When a program is ported from GNAT to some other Ada compiler, no special attention is required unless the shared=XXX form parameter is used in the program.  In this case, you must examine the documentation of the new compiler to see if it supports the required file sharing semantics, and form strings modified appropriately.  Of course it may be the case that the program cannot be ported if the target compiler does not support the required functionality.  The best approach in writing portable code is to avoid file sharing (and hence the use of the shared=XXX parameter in the form string) completely.

One common use of file sharing in Ada 83 is the use of instantiations of Sequential_IO on the same file with different types, to achieve heterogeneous input-output.  Although this approach will work in GNAT if shared=yes is specified, it is preferable in Ada 95 to use Stream_IO for this purpose (using the stream attributes)

Open Modes
==========

Open and Create calls result in a call to fopen using the mode shown in the following table:

Open and Create Call Modes

                                    OPEN            CREATE
     Append_File                    "r+"             "w+"
     In_File                        "r"              "w+"
     Out_File (Direct_IO)           "r+"             "w"
     Out_File (all other cases)     "w"              "w"
     Inout_File                     "r+"             "w+"

If text file translation is required, then either b or t is added to the mode, depending on the setting of Text.  Text file translation refers to the mapping of CR/LF sequences in an external file to LF characters internally.  This mapping only occurs in DOS and DOS-like systems, and is not relevant to other systems.

A special case occurs with Stream_IO.  As shown in the above table, the file is initially opened in r or w mode for the In_File and Out_File cases.  If a Set_Mode operation subsequently requires switching from reading to writing or vice-versa, then the file is reopened in r+ mode to permit the required operation.

Operations on C Streams
=======================

The package Interfaces.C_Streams provides an Ada program with direct access to the C library functions for operations on C streams:

     package Interfaces.C_Streams is
       -- Note: the reason we do not use the types that are in
       -- Interfaces.C is that we want to avoid dragging in the
       -- code in this unit if possible.
       subtype chars is System.Address;
       -- Pointer to null-terminated array of characters
       subtype FILEs is System.Address;
       -- Corresponds to the C type FILE*
       subtype voids is System.Address;
       -- Corresponds to the C type void*
       subtype int is Integer;
       subtype long is Long_Integer;
       -- Note: the above types are subtypes deliberately, and it
       -- is part of this spec that the above correspondences are
       -- guaranteed.  This means that it is legitimate to, for
       -- example, use Integer instead of int.  We provide these
       -- synonyms for clarity, but in some cases it may be
       -- convenient to use the underlying types (for example to
       -- avoid an unnecessary dependency of a spec on the spec
       -- of this unit).
       type size_t is mod 2 ** Standard'Address_Size;
       NULL_Stream : constant FILEs;
       -- Value returned (NULL in C) to indicate an
       -- fdopen/fopen/tmpfile error
       ----------------------------------
       -- Constants Defined in stdio.h --
       ----------------------------------
       EOF : constant int;
       -- Used by a number of routines to indicate error or
       -- end of file
       IOFBF : constant int;
       IOLBF : constant int;
       IONBF : constant int;
       -- Used to indicate buffering mode for setvbuf call
       SEEK_CUR : constant int;
       SEEK_END : constant int;
       SEEK_SET : constant int;
       -- Used to indicate origin for fseek call
       function stdin return FILEs;
       function stdout return FILEs;
       function stderr return FILEs;
       -- Streams associated with standard files
       --------------------------
       -- Standard C functions --
       --------------------------
       -- The functions selected below are ones that are
       -- available in DOS, OS/2, UNIX and Xenix (but not
       -- necessarily in ANSI C).  These are very thin interfaces
       -- which copy exactly the C headers.  For more
       -- documentation on these functions, see the Microsoft C
       -- "Run-Time Library Reference" (Microsoft Press, 1990,
       -- ISBN 1-55615-225-6), which includes useful information
       -- on system compatibility.
       procedure clearerr (stream : FILEs);
       function fclose (stream : FILEs) return int;
       function fdopen (handle : int; mode : chars) return FILEs;
       function feof (stream : FILEs) return int;
       function ferror (stream : FILEs) return int;
       function fflush (stream : FILEs) return int;
       function fgetc (stream : FILEs) return int;
       function fgets (strng : chars; n : int; stream : FILEs)
           return chars;
       function fileno (stream : FILEs) return int;
       function fopen (filename : chars; Mode : chars)
           return FILEs;
       -- Note: to maintain target independence, use
       -- text_translation_required, a boolean variable defined in
       -- a-sysdep.c to deal with the target dependent text
       -- translation requirement.  If this variable is set,
       -- then  b/t should be appended to the standard mode
       -- argument to set the text translation mode off or on
       -- as required.
       function fputc (C : int; stream : FILEs) return int;
       function fputs (Strng : chars; Stream : FILEs) return int;
       function fread
          (buffer : voids;
           size : size_t;
           count : size_t;
           stream : FILEs)
           return size_t;
       function freopen
          (filename : chars;
           mode : chars;
           stream : FILEs)
           return FILEs;
       function fseek
          (stream : FILEs;
           offset : long;
           origin : int)
           return int;
       function ftell (stream : FILEs) return long;
       function fwrite
          (buffer : voids;
           size : size_t;
           count : size_t;
           stream : FILEs)
           return size_t;
       function isatty (handle : int) return int;
       procedure mktemp (template : chars);
       -- The return value (which is just a pointer to template)
       -- is discarded
       procedure rewind (stream : FILEs);
       function rmtmp return int;
       function setvbuf
          (stream : FILEs;
           buffer : chars;
           mode : int;
           size : size_t)
           return int;

       function tmpfile return FILEs;
       function ungetc (c : int; stream : FILEs) return int;
       function unlink (filename : chars) return int;
       ---------------------
       -- Extra functions --
       ---------------------
       -- These functions supply slightly thicker bindings than
       -- those above.  They are derived from functions in the
       -- C Run-Time Library, but may do a bit more work than
       -- just directly calling one of the Library functions.
       function is_regular_file (handle : int) return int;
       -- Tests if given handle is for a regular file (result 1)
       -- or for a non-regular file (pipe or device, result 0).
       ---------------------------------
       -- Control of Text/Binary Mode --
       ---------------------------------
       -- If text_translation_required is true, then the following
       -- functions may be used to dynamically switch a file from
       -- binary to text mode or vice versa.  These functions have
       -- no effect if text_translation_required is false (i.e.  in
       -- normal UNIX mode).  Use fileno to get a stream handle.
       procedure set_binary_mode (handle : int);
       procedure set_text_mode (handle : int);
       ----------------------------
       -- Full Path Name support --
       ----------------------------
       procedure full_name (nam : chars; buffer : chars);
       -- Given a NUL terminated string representing a file
       -- name, returns in buffer a NUL terminated string
       -- representing the full path name for the file name.
       -- On systems where it is relevant the   drive is also
       -- part of the full path name.  It is the responsibility
       -- of the caller to pass an actual parameter for buffer
       -- that is big enough for any full path name.  Use
       -- max_path_len given below as the size of buffer.
       max_path_len : integer;
       -- Maximum length of an allowable full path name on the
       -- system, including a terminating NUL character.
     end Interfaces.C_Streams;

Interfacing to C Streams
========================

The packages in this section permit interfacing Ada files to C Stream operations.

      with Interfaces.C_Streams;
      package Ada.Sequential_IO.C_Streams is
         function C_Stream (F : File_Type)
            return Interfaces.C_Streams.FILEs;
         procedure Open
           (File : in out File_Type;
            Mode : in File_Mode;
            C_Stream : in Interfaces.C_Streams.FILEs;
            Form : in String := "");
      end Ada.Sequential_IO.C_Streams;

       with Interfaces.C_Streams;
       package Ada.Direct_IO.C_Streams is
          function C_Stream (F : File_Type)
             return Interfaces.C_Streams.FILEs;
          procedure Open
            (File : in out File_Type;
             Mode : in File_Mode;
             C_Stream : in Interfaces.C_Streams.FILEs;
             Form : in String := "");
       end Ada.Direct_IO.C_Streams;

       with Interfaces.C_Streams;
       package Ada.Text_IO.C_Streams is
          function C_Stream (F : File_Type)
             return Interfaces.C_Streams.FILEs;
          procedure Open
            (File : in out File_Type;
             Mode : in File_Mode;
             C_Stream : in Interfaces.C_Streams.FILEs;
             Form : in String := "");
       end Ada.Text_IO.C_Streams;

       with Interfaces.C_Streams;
       package Ada.Wide_Text_IO.C_Streams is
          function C_Stream (F : File_Type)
             return Interfaces.C_Streams.FILEs;
          procedure Open
            (File : in out File_Type;
             Mode : in File_Mode;
             C_Stream : in Interfaces.C_Streams.FILEs;
             Form : in String := "");
      end Ada.Wide_Text_IO.C_Streams;

       with Interfaces.C_Streams;
       package Ada.Wide_Wide_Text_IO.C_Streams is
          function C_Stream (F : File_Type)
             return Interfaces.C_Streams.FILEs;
          procedure Open
            (File : in out File_Type;
             Mode : in File_Mode;
             C_Stream : in Interfaces.C_Streams.FILEs;
             Form : in String := "");
      end Ada.Wide_Wide_Text_IO.C_Streams;

      with Interfaces.C_Streams;
      package Ada.Stream_IO.C_Streams is
         function C_Stream (F : File_Type)
            return Interfaces.C_Streams.FILEs;
         procedure Open
           (File : in out File_Type;
            Mode : in File_Mode;
            C_Stream : in Interfaces.C_Streams.FILEs;
            Form : in String := "");
      end Ada.Stream_IO.C_Streams;

In each of these six packages, the C_Stream function obtains the FILE pointer from a currently opened Ada file.  It is then possible to use the Interfaces.C_Streams package to operate on this stream, or the stream can be passed to a C program which can operate on it directly.  Of course the program is responsible for ensuring that only appropriate sequences of operations are executed.

One particular use of relevance to an Ada program is that the setvbuf function can be used to control the buffering of the stream used by an Ada file.  In the absence of such a call the standard default buffering is used.

The Open procedures in these packages open a file giving an existing C Stream instead of a file name.  Typically this stream is imported from a C program, allowing an Ada file to operate on an existing C file.

The GNAT Library
****************

The GNAT library contains a number of general and special purpose packages. It represents functionality that the GNAT developers have found useful, and which is made available to GNAT users.  The packages described here are fully supported, and upwards compatibility will be maintained in future releases, so you can use these facilities with the confidence that the same functionality will be available in future releases.

The chapter here simply gives a brief summary of the facilities available. The full documentation is found in the spec file for the package.  The full sources of these library packages, including both spec and body, are provided with all GNAT releases.  For example, to find out the full specifications of the SPITBOL pattern matching capability, including a full tutorial and extensive examples, look in the g-spipat.ads file in the library.

For each entry here, the package name (as it would appear in a with clause) is given, followed by the name of the corresponding spec file in parentheses.  The packages are children in four hierarchies, Ada, Interfaces, System, and GNAT, the latter being a GNAT-specific hierarchy.

Note that an application program should only use packages in one of these four hierarchies if the package is defined in the Ada Reference Manual, or is listed in this section of the GNAT Programmers Reference Manual. All other units should be considered internal implementation units and should not be directly with'ed by application code.  The use of a with statement that references one of these internal implementation units makes an application potentially dependent on changes in versions of GNAT, and will generate a warning message.

Ada.Characters.Latin_9 (a-chlat9.ads)
=====================================

This child of Ada.Characters provides a set of definitions corresponding to those in the RM-defined package Ada.Characters.Latin_1 but with the few modifications required for Latin-9 The provision of such a package is specifically authorized by the Ada Reference Manual (RM A.3(27)).

Ada.Characters.Wide_Latin_1 (a-cwila1.ads)
==========================================

This child of Ada.Characters provides a set of definitions corresponding to those in the RM-defined package Ada.Characters.Latin_1 but with the types of the constants being Wide_Character instead of Character.  The provision of such a package is specifically authorized by the Ada Reference Manual (RM A.3(27)).

Ada.Characters.Wide_Latin_9 (a-cwila1.ads)
==========================================

This child of Ada.Characters provides a set of definitions corresponding to those in the GNAT defined package Ada.Characters.Latin_9 but with the types of the constants being Wide_Character instead of Character.  The provision of such a package is specifically authorized by the Ada Reference Manual (RM A.3(27)).

Ada.Characters.Wide_Wide_Latin_1 (a-czila1.ads)
===============================================

This child of Ada.Characters provides a set of definitions corresponding to those in the RM-defined package Ada.Characters.Latin_1 but with the types of the constants being Wide_Wide_Character instead of Character.  The provision of such a package is specifically authorized by the Ada Reference Manual (RM A.3(27)).

Ada.Characters.Wide_Wide_Latin_9 (a-czila9.ads)
===============================================

This child of Ada.Characters provides a set of definitions corresponding to those in the GNAT defined package Ada.Characters.Latin_9 but with the types of the constants being Wide_Wide_Character instead of Character.  The provision of such a package is specifically authorized by the Ada Reference Manual (RM A.3(27)).

Ada.Command_Line.Remove (a-colire.ads)
======================================

This child of Ada.Command_Line provides a mechanism for logically removing arguments from the argument list.  Once removed, an argument is not visible to further calls on the subprograms in Ada.Command_Line will not see the removed argument.

Ada.Command_Line.Environment (a-colien.ads)
===========================================

This child of Ada.Command_Line provides a mechanism for obtaining environment values on systems where this concept makes sense.

Ada.Direct_IO.C_Streams (a-diocst.ads)
======================================

This package provides subprograms that allow interfacing between C streams and Direct_IO.  The stream identifier can be extracted from a file opened on the Ada side, and an Ada file can be constructed from a stream opened on the C side.

Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads)
================================================

This child subprogram provides a way of testing for the null exception occurrence (Null_Occurrence) without raising an exception.

Ada.Exceptions.Traceback (a-exctra.ads)
=======================================

This child package provides the subprogram (Tracebacks) to give a traceback array of addresses based on an exception occurrence.

Ada.Sequential_IO.C_Streams (a-siocst.ads)
==========================================

This package provides subprograms that allow interfacing between C streams and Sequential_IO.  The stream identifier can be extracted from a file opened on the Ada side, and an Ada file can be constructed from a stream opened on the C side.

Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads)
==============================================

This package provides subprograms that allow interfacing between C streams and Stream_IO.  The stream identifier can be extracted from a file opened on the Ada side, and an Ada file can be constructed from a stream opened on the C side.

Ada.Strings.Unbounded.Text_IO (a-suteio.ads)
============================================

This package provides subprograms for Text_IO for unbounded strings, avoiding the necessity for an intermediate operation with ordinary strings.

Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads)
======================================================

This package provides subprograms for Text_IO for unbounded wide strings, avoiding the necessity for an intermediate operation with ordinary wide strings.

Ada.Strings.Wide_Wide_Unbounded.Wide_Wide_Text_IO (a-szuzti.ads)
================================================================

This package provides subprograms for Text_IO for unbounded wide wide strings, avoiding the necessity for an intermediate operation with ordinary wide wide strings.

Ada.Text_IO.C_Streams (a-tiocst.ads)
====================================

This package provides subprograms that allow interfacing between C streams and Text_IO.  The stream identifier can be extracted from a file opened on the Ada side, and an Ada file can be constructed from a stream opened on the C side.

Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads)
=========================================

This package provides subprograms that allow interfacing between C streams and Wide_Text_IO.  The stream identifier can be extracted from a file opened on the Ada side, and an Ada file can be constructed from a stream opened on the C side.

Ada.Wide_Wide_Text_IO.C_Streams (a-ztcstr.ads)
==============================================

This package provides subprograms that allow interfacing between C streams and Wide_Wide_Text_IO.  The stream identifier can be extracted from a file opened on the Ada side, and an Ada file can be constructed from a stream opened on the C side.

GNAT.Array_Split (g-arrspl.ads)
===============================

Useful array-manipulation routines: given a set of separators, split an array wherever the separators appear, and provide direct access to the resulting slices.

GNAT.AWK (g-awk.ads)
====================

Provides AWK-like parsing functions, with an easy interface for parsing one or more files containing formatted data.  The file is viewed as a database where each record is a line and a field is a data element in this line.

GNAT.Bounded_Buffers (g-boubuf.ads)
===================================

Provides a concurrent generic bounded buffer abstraction.  Instances are useful directly or as parts of the implementations of other abstractions, such as mailboxes.

GNAT.Bounded_Mailboxes (g-boumai.ads)
=====================================

Provides a thread-safe asynchronous intertask mailbox communication facility.

GNAT.Bubble_Sort (g-bubsor.ads)
===============================

Provides a general implementation of bubble sort usable for sorting arbitrary data items.  Exchange and comparison procedures are provided by passing access-to-procedure values.

GNAT.Bubble_Sort_A (g-busora.ads)
=================================

Provides a general implementation of bubble sort usable for sorting arbitrary data items.  Move and comparison procedures are provided by passing access-to-procedure values. This is an older version, retained for compatibility. Usually GNAT.Bubble_Sort will be preferable.

GNAT.Bubble_Sort_G (g-busorg.ads)
=================================

Similar to Bubble_Sort_A except that the move and sorting procedures are provided as generic parameters, this improves efficiency, especially if the procedures can be inlined, at the expense of duplicating code for multiple instantiations.

GNAT.Calendar (g-calend.ads)
============================

Extends the facilities provided by Ada.Calendar to include handling of days of the week, an extended Split and Time_Of capability. Also provides conversion of Ada.Calendar.Time values to and from the C timeval format.

GNAT.Calendar.Time_IO (g-catiio.ads)
====================================

GNAT.CRC32 (g-crc32.ads)
========================

This package implements the CRC-32 algorithm.  For a full description of this algorithm see ``Computation of Cyclic Redundancy Checks via Table Look-Up'', Communications of the ACM, Vol. 31 No. 8, pp. 1008-1013, Aug. 1988.  Sarwate, D.V.

Provides an extended capability for formatted output of time values with full user control over the format.  Modeled on the GNU Date specification.

GNAT.Case_Util (g-casuti.ads)
=============================

A set of simple routines for handling upper and lower casing of strings without the overhead of the full casing tables in Ada.Characters.Handling.

GNAT.CGI (g-cgi.ads)
====================

This is a package for interfacing a GNAT program with a Web server via the Common Gateway Interface (CGI).  Basically this package parses the CGI parameters, which are a set of key/value pairs sent by the Web server.  It builds a table whose index is the key and provides some services to deal with this table.

GNAT.CGI.Cookie (g-cgicoo.ads)
==============================

This is a package to interface a GNAT program with a Web server via the Common Gateway Interface (CGI).  It exports services to deal with Web cookies (piece of information kept in the Web client software).

GNAT.CGI.Debug (g-cgideb.ads)
=============================

This is a package to help debugging CGI (Common Gateway Interface) programs written in Ada.

GNAT.Command_Line (g-comlin.ads)
================================

Provides a high level interface to Ada.Command_Line facilities, including the ability to scan for named switches with optional parameters and expand file names using wild card notations.

GNAT.Compiler_Version (g-comver.ads)
====================================

Provides a routine for obtaining the version of the compiler used to compile the program. More accurately this is the version of the binder used to bind the program (this will normally be the same as the version of the compiler if a consistent tool set is used to compile all units of a partition).

GNAT.Ctrl_C (g-ctrl_c.ads)
==========================

Provides a simple interface to handle Ctrl-C keyboard events.

GNAT.Current_Exception (g-curexc.ads)
=====================================

Provides access to information on the current exception that has been raised without the need for using the Ada-95 exception choice parameter specification syntax.  This is particularly useful in simulating typical facilities for obtaining information about exceptions provided by Ada 83 compilers.

GNAT.Debug_Pools (g-debpoo.ads)
===============================

Provide a debugging storage pools that helps tracking memory corruption problems.  See section ``Finding memory problems with GNAT Debug Pool'' in the GNAT User's Guide.

GNAT.Debug_Utilities (g-debuti.ads)
===================================

Provides a few useful utilities for debugging purposes, including conversion to and from string images of address values. Supports both C and Ada formats for hexadecimal literals.

GNAT.Directory_Operations (g-dirope.ads)
========================================

Provides a set of routines for manipulating directories, including changing the current directory, making new directories, and scanning the files in a directory.

GNAT.Dynamic_HTables (g-dynhta.ads)
===================================

A generic implementation of hash tables that can be used to hash arbitrary data.  Provided in two forms, a simple form with built in hash functions, and a more complex form in which the hash function is supplied.

This package provides a facility similar to that of GNAT.HTable, except that this package declares a type that can be used to define dynamic instances of the hash table, while an instantiation of GNAT.HTable creates a single instance of the hash table.

GNAT.Dynamic_Tables (g-dyntab.ads)
==================================

A generic package providing a single dimension array abstraction where the length of the array can be dynamically modified.

This package provides a facility similar to that of GNAT.Table, except that this package declares a type that can be used to define dynamic instances of the table, while an instantiation of GNAT.Table creates a single instance of the table type.

GNAT.Exception_Actions (g-excact.ads)
=====================================

Provides callbacks when an exception is raised. Callbacks can be registered for specific exceptions, or when any exception is raised. This can be used for instance to force a core dump to ease debugging.

GNAT.Exception_Traces (g-exctra.ads)
====================================

Provides an interface allowing to control automatic output upon exception occurrences.

GNAT.Exceptions (g-expect.ads)
==============================

Normally it is not possible to raise an exception with a message from a subprogram in a pure package, since the necessary types and subprograms are in Ada.Exceptions which is not a pure unit. GNAT.Exceptions provides a facility for getting around this limitation for a few predefined exceptions, and for example allow raising Constraint_Error with a message from a pure subprogram.

GNAT.Expect (g-expect.ads)
==========================

Provides a set of subprograms similar to what is available with the standard Tcl Expect tool. It allows you to easily spawn and communicate with an external process. You can send commands or inputs to the process, and compare the output with some expected regular expression. Currently GNAT.Expect is implemented on all native GNAT ports except for OpenVMS. It is not implemented for cross ports, and in particular is not implemented for VxWorks or LynxOS.

GNAT.Float_Control (g-flocon.ads)
=================================

Provides an interface for resetting the floating-point processor into the mode required for correct semantic operation in Ada.  Some third party library calls may cause this mode to be modified, and the Reset procedure in this package can be used to reestablish the required mode.

GNAT.Heap_Sort (g-heasor.ads)
=============================

Provides a general implementation of heap sort usable for sorting arbitrary data items. Exchange and comparison procedures are provided by passing access-to-procedure values.  The algorithm used is a modified heap sort that performs approximately N*log(N) comparisons in the worst case.

GNAT.Heap_Sort_A (g-hesora.ads)
===============================

Provides a general implementation of heap sort usable for sorting arbitrary data items. Move and comparison procedures are provided by passing access-to-procedure values.  The algorithm used is a modified heap sort that performs approximately N*log(N) comparisons in the worst case. This differs from GNAT.Heap_Sort in having a less convenient interface, but may be slightly more efficient.

GNAT.Heap_Sort_G (g-hesorg.ads)
===============================

Similar to Heap_Sort_A except that the move and sorting procedures are provided as generic parameters, this improves efficiency, especially if the procedures can be inlined, at the expense of duplicating code for multiple instantiations.

GNAT.HTable (g-htable.ads)
==========================

A generic implementation of hash tables that can be used to hash arbitrary data.  Provides two approaches, one a simple static approach, and the other allowing arbitrary dynamic hash tables.

GNAT.IO (g-io.ads)
==================

A simple preelaborable input-output package that provides a subset of simple Text_IO functions for reading characters and strings from Standard_Input, and writing characters, strings and integers to either Standard_Output or Standard_Error.

GNAT.IO_Aux (g-io_aux.ads)
==========================

Provides some auxiliary functions for use with Text_IO, including a test for whether a file exists, and functions for reading a line of text.

GNAT.Lock_Files (g-locfil.ads)
==============================

Provides a general interface for using files as locks.  Can be used for providing program level synchronization.

GNAT.MD5 (g-md5.ads)
====================

Implements the MD5 Message-Digest Algorithm as described in RFC 1321.

GNAT.Memory_Dump (g-memdum.ads)
===============================

Provides a convenient routine for dumping raw memory to either the standard output or standard error files. Uses GNAT.IO for actual output.

GNAT.Most_Recent_Exception (g-moreex.ads)
=========================================

Provides access to the most recently raised exception.  Can be used for various logging purposes, including duplicating functionality of some Ada 83 implementation dependent extensions.

GNAT.OS_Lib (g-os_lib.ads)
==========================

Provides a range of target independent operating system interface functions, including time/date management, file operations, subprocess management, including a portable spawn procedure, and access to environment variables and error return codes.

GNAT.Perfect_Hash_Generators (g-pehage.ads)
===========================================

Provides a generator of static minimal perfect hash functions. No collisions occur and each item can be retrieved from the table in one probe (perfect property). The hash table size corresponds to the exact size of the key set and no larger (minimal property). The key set has to be know in advance (static property). The hash functions are also order preservering. If w2 is inserted after w1 in the generator, their hashcode are in the same order. These hashing functions are very convenient for use with realtime applications.

GNAT.Regexp (g-regexp.ads)
==========================

A simple implementation of regular expressions, using a subset of regular expression syntax copied from familiar Unix style utilities.  This is the simples of the three pattern matching packages provided, and is particularly suitable for ``file globbing'' applications.

GNAT.Registry (g-regist.ads)
============================

This is a high level binding to the Windows registry.  It is possible to do simple things like reading a key value, creating a new key.  For full registry API, but at a lower level of abstraction, refer to the Win32.Winreg package provided with the Win32Ada binding

GNAT.Regpat (g-regpat.ads)
==========================

A complete implementation of Unix-style regular expression matching, copied from the original V7 style regular expression library written in C by Henry Spencer (and binary compatible with this C library).

GNAT.Secondary_Stack_Info (g-sestin.ads)
========================================

Provide the capability to query the high water mark of the current task's secondary stack.

GNAT.Semaphores (g-semaph.ads)
==============================

Provides classic counting and binary semaphores using protected types.

GNAT.Signals (g-signal.ads)
===========================

Provides the ability to manipulate the blocked status of signals on supported targets.

GNAT.Sockets (g-socket.ads)
===========================

A high level and portable interface to develop sockets based applications. This package is based on the sockets thin binding found in GNAT.Sockets.Thin. Currently GNAT.Sockets is implemented on all native GNAT ports except for OpenVMS.  It is not implemented for the LynxOS cross port.

GNAT.Source_Info (g-souinf.ads)
===============================

Provides subprograms that give access to source code information known at compile time, such as the current file name and line number.

GNAT.Spell_Checker (g-speche.ads)
=================================

Provides a function for determining whether one string is a plausible near misspelling of another string.

GNAT.Spitbol.Patterns (g-spipat.ads)
====================================

A complete implementation of SNOBOL4 style pattern matching.  This is the most elaborate of the pattern matching packages provided.  It fully duplicates the SNOBOL4 dynamic pattern construction and matching capabilities, using the efficient algorithm developed by Robert Dewar for the SPITBOL system.

GNAT.Spitbol (g-spitbo.ads)
===========================

The top level package of the collection of SPITBOL-style functionality, this package provides basic SNOBOL4 string manipulation functions, such as Pad, Reverse, Trim, Substr capability, as well as a generic table function useful for constructing arbitrary mappings from strings in the style of the SNOBOL4 TABLE function.

GNAT.Spitbol.Table_Boolean (g-sptabo.ads)
=========================================

A library level of instantiation of GNAT.Spitbol.Patterns.Table for type Standard.Boolean, giving an implementation of sets of string values.

GNAT.Spitbol.Table_Integer (g-sptain.ads)
=========================================

A library level of instantiation of GNAT.Spitbol.Patterns.Table for type Standard.Integer, giving an implementation of maps from string to integer values.

GNAT.Spitbol.Table_VString (g-sptavs.ads)
=========================================

A library level of instantiation of GNAT.Spitbol.Patterns.Table for a variable length string type, giving an implementation of general maps from strings to strings.

GNAT.Strings (g-string.ads)
===========================

Common String access types and related subprograms. Basically it defines a string access and an array of string access types.

GNAT.String_Split (g-strspl.ads)
================================

Useful string manipulation routines: given a set of separators, split a string wherever the separators appear, and provide direct access to the resulting slices. This package is instantiated from GNAT.Array_Split.

GNAT.Table (g-table.ads)
========================

A generic package providing a single dimension array abstraction where the length of the array can be dynamically modified.

This package provides a facility similar to that of GNAT.Dynamic_Tables, except that this package declares a single instance of the table type, while an instantiation of GNAT.Dynamic_Tables creates a type that can be used to define dynamic instances of the table.

GNAT.Task_Lock (g-tasloc.ads)
=============================

A very simple facility for locking and unlocking sections of code using a single global task lock.  Appropriate for use in situations where contention between tasks is very rarely expected.

GNAT.Threads (g-thread.ads)
===========================

Provides facilities for creating and destroying threads with explicit calls. These threads are known to the GNAT run-time system.  These subprograms are exported C-convention procedures intended to be called from foreign code. By using these primitives rather than directly calling operating systems routines, compatibility with the Ada tasking runt-time is provided.

GNAT.Traceback (g-traceb.ads)
=============================

Provides a facility for obtaining non-symbolic traceback information, useful in various debugging situations.

GNAT.Traceback.Symbolic (g-trasym.ads)
======================================

Provides symbolic traceback information that includes the subprogram name and line number information.

GNAT.Wide_String_Split (g-wistsp.ads)
=====================================

Useful wide string manipulation routines: given a set of separators, split a wide string wherever the separators appear, and provide direct access to the resulting slices. This package is instantiated from GNAT.Array_Split.

GNAT.Wide_Wide_String_Split (g-zistsp.ads)
==========================================

Useful wide wide string manipulation routines: given a set of separators, split a wide wide string wherever the separators appear, and provide direct access to the resulting slices. This package is instantiated from GNAT.Array_Split.

Interfaces.C.Extensions (i-cexten.ads)
======================================

This package contains additional C-related definitions, intended for use with either manually or automatically generated bindings to C libraries.

Interfaces.C.Streams (i-cstrea.ads)
===================================

This package is a binding for the most commonly used operations on C streams.

Interfaces.CPP (i-cpp.ads)
==========================

This package provides facilities for use in interfacing to C++.  It is primarily intended to be used in connection with automated tools for the generation of C++ interfaces.

Interfaces.Os2lib (i-os2lib.ads)
================================

This package provides interface definitions to the OS/2 library. It is a thin binding which is a direct translation of the various <bse.h> files.

Interfaces.Os2lib.Errors (i-os2err.ads)
=======================================

This package provides definitions of the OS/2 error codes.

Interfaces.Os2lib.Synchronization (i-os2syn.ads)
================================================

This is a child package that provides definitions for interfacing to the OS/2 synchronization primitives.

Interfaces.Os2lib.Threads (i-os2thr.ads)
========================================

This is a child package that provides definitions for interfacing to the OS/2 thread primitives.

Interfaces.Packed_Decimal (i-pacdec.ads)
========================================

This package provides a set of routines for conversions to and from a packed decimal format compatible with that used on IBM mainframes.

Interfaces.VxWorks (i-vxwork.ads)
=================================

This package provides a limited binding to the VxWorks API. In particular, it interfaces with the VxWorks hardware interrupt facilities.

Interfaces.VxWorks.IO (i-vxwoio.ads)
====================================

This package provides a binding to the ioctl (IO/Control) function of VxWorks, defining a set of option values and function codes. A particular use of this package is to enable the use of Get_Immediate under VxWorks.

System.Address_Image (s-addima.ads)
===================================

This function provides a useful debugging function that gives an (implementation dependent) string which identifies an address.

System.Assertions (s-assert.ads)
================================

This package provides the declaration of the exception raised by an run-time assertion failure, as well as the routine that is used internally to raise this assertion.

System.Memory (s-memory.ads)
============================

This package provides the interface to the low level routines used by the generated code for allocation and freeing storage for the default storage pool (analogous to the C routines malloc and free. It also provides a reallocation interface analogous to the C routine realloc. The body of this unit may be modified to provide alternative allocation mechanisms for the default pool, and in addition, direct calls to this unit may be made for low level allocation uses (for example see the body of GNAT.Tables).

System.Partition_Interface (s-parint.ads)
=========================================

This package provides facilities for partition interfacing.  It is used primarily in a distribution context when using Annex E with GLADE.

System.Restrictions (s-restri.ads)
==================================

This package provides facilities for accessing at run-time the status of restrictions specified at compile time for the partition. Information is available both with regard to actual restrictions specified, and with regard to compiler determined information on which restrictions are violated by one or more packages in the partition.

System.Rident (s-rident.ads)
============================

This package provides definitions of the restrictions identifiers supported by GNAT, and also the format of the restrictions provided in package System.Restrictions. It is not normally necessary to with this generic package since the necessary instantiation is included in package System.Restrictions.

System.Task_Info (s-tasinf.ads)
===============================

This package provides target dependent functionality that is used to support the Task_Info pragma

System.Wch_Cnv (s-wchcnv.ads)
=============================

This package provides routines for converting between wide and wide wide characters and a representation as a value of type Standard.String, using a specified wide character encoding method.  It uses definitions in package System.Wch_Con.

System.Wch_Con (s-wchcon.ads)
=============================

This package provides definitions and descriptions of the various methods used for encoding wide characters in ordinary strings.  These definitions are used by the package System.Wch_Cnv.

Interfacing to Other Languages
******************************

The facilities in annex B of the Ada 95 Reference Manual are fully implemented in GNAT, and in addition, a full interface to C++ is provided.

Interfacing to C
================

Interfacing to C with GNAT can use one of two approaches:

  * The types in the package Interfaces.C may be used.

  * Standard Ada types may be used directly.  This may be less portable to other compilers, but will work on all GNAT compilers, which guarantee correspondence between the C and Ada types.

Pragma Convention C may be applied to Ada types, but mostly has no effect, since this is the default.  The following table shows the correspondence between Ada scalar types and the corresponding C types.

Integer
     int

Short_Integer
     short

Short_Short_Integer
     signed char

Long_Integer
     long

Long_Long_Integer
     long long

Short_Float
     float

Float
     float

Long_Float
     double

Long_Long_Float
     This is the longest floating-point type supported by the hardware.

Additionally, there are the following general correspondences between Ada and C types:

  * Ada enumeration types map to C enumeration types directly if pragma Convention C is specified, which causes them to have int length.  Without pragma Convention C, Ada enumeration types map to 8, 16, or 32 bits (i.e. C types signed char, short, int, respectively) depending on the number of values passed. This is the only case in which pragma Convention C affects the representation of an Ada type.

  * Ada access types map to C pointers, except for the case of pointers to unconstrained types in Ada, which have no direct C equivalent.

  * Ada arrays map directly to C arrays.

  * Ada records map directly to C structures.

  * Packed Ada records map to C structures where all members are bit fields of the length corresponding to the TYPE'Size value in Ada.

Interfacing to C++
==================

The interface to C++ makes use of the following pragmas, which are primarily intended to be constructed automatically using a binding generator tool, although it is possible to construct them by hand.  No suitable binding generator tool is supplied with GNAT though.

Using these pragmas it is possible to achieve complete inter-operability between Ada tagged types and C class definitions. See Implementation Defined Pragmas, for more details.

pragma CPP_Class ([Entity =>] LOCAL_NAME)
     The argument denotes an entity in the current declarative region that is declared as a tagged or untagged record type.  It indicates that the type corresponds to an externally declared C++ class type, and is to be laid out the same way that C++ would lay out the type.

pragma CPP_Constructor ([Entity =>] LOCAL_NAME)
     This pragma identifies an imported function (imported in the usual way with pragma Import) as corresponding to a C++ constructor.

pragma CPP_Vtable ...
     One CPP_Vtable pragma can be present for each component of type CPP.Interfaces.Vtable_Ptr in a record to which pragma CPP_Class applies.

Interfacing to COBOL
====================

Interfacing to COBOL is achieved as described in section B.4 of the Ada 95 reference manual.

Interfacing to Fortran
======================

Interfacing to Fortran is achieved as described in section B.5 of the reference manual.  The pragma Convention Fortran, applied to a multi-dimensional array causes the array to be stored in column-major order as required for convenient interface to Fortran.

Interfacing to non-GNAT Ada code
================================

It is possible to specify the convention Ada in a pragma Import or pragma Export.  However this refers to the calling conventions used by GNAT, which may or may not be similar enough to those used by some other Ada 83 or Ada 95 compiler to allow interoperation.

If arguments types are kept simple, and if the foreign compiler generally follows system calling conventions, then it may be possible to integrate files compiled by other Ada compilers, provided that the elaboration issues are adequately addressed (for example by eliminating the need for any load time elaboration).

In particular, GNAT running on VMS is designed to be highly compatible with the DEC Ada 83 compiler, so this is one case in which it is possible to import foreign units of this type, provided that the data items passed are restricted to simple scalar values or simple record types without variants, or simple array types with fixed bounds.

Specialized Needs Annexes
*************************

Ada 95 defines a number of specialized needs annexes, which are not required in all implementations.  However, as described in this chapter, GNAT implements all of these special needs annexes:

Systems Programming (Annex C)
     The Systems Programming Annex is fully implemented.

Real-Time Systems (Annex D)
     The Real-Time Systems Annex is fully implemented.

Distributed Systems (Annex E)
     Stub generation is fully implemented in the GNAT compiler.  In addition, a complete compatible PCS is available as part of the GLADE system, a separate product.  When the two products are used in conjunction, this annex is fully implemented.

Information Systems (Annex F)
     The Information Systems annex is fully implemented.

Numerics (Annex G)
     The Numerics Annex is fully implemented.

Safety and Security (Annex H)
     The Safety and Security annex is fully implemented.

Implementation of Specific Ada Features
***************************************

This chapter describes the GNAT implementation of several Ada language facilities.

Machine Code Insertions
=======================

Package Machine_Code provides machine code support as described in the Ada 95 Reference Manual in two separate forms:

  * Machine code statements, consisting of qualified expressions that fit the requirements of RM section 13.8.

  * An intrinsic callable procedure, providing an alternative mechanism of including machine instructions in a subprogram.

The two features are similar, and both are closely related to the mechanism provided by the asm instruction in the GNU C compiler.  Full understanding and use of the facilities in this package requires understanding the asm instruction as described in Using the GNU Compiler Collection (GCC) by Richard Stallman. The relevant section is titled ``Extensions to the C Language Family'' -> ``Assembler Instructions with C Expression Operands''.

Calls to the function Asm and the procedure Asm have identical semantic restrictions and effects as described below.  Both are provided so that the procedure call can be used as a statement, and the function call can be used to form a code_statement.

The first example given in the GCC documentation is the C asm instruction:

        asm ("fsinx %1 %0" : "=f" (result) : "f" (angle));

The equivalent can be written for GNAT as:

     Asm ("fsinx %1 %0",
          My_Float'Asm_Output ("=f", result),
          My_Float'Asm_Input  ("f",  angle));

The first argument to Asm is the assembler template, and is identical to what is used in GNU C.  This string must be a static expression.  The second argument is the output operand list.  It is either a single Asm_Output attribute reference, or a list of such references enclosed in parentheses (technically an array aggregate of such references).

The Asm_Output attribute denotes a function that takes two parameters.  The first is a string, the second is the name of a variable of the type designated by the attribute prefix.  The first (string) argument is required to be a static expression and designates the constraint for the parameter (e.g. what kind of register is required).  The second argument is the variable to be updated with the result.  The possible values for constraint are the same as those used in the RTL, and are dependent on the configuration file used to build the GCC back end.  If there are no output operands, then this argument may either be omitted, or explicitly given as No_Output_Operands.

The second argument of MY_FLOAT'Asm_Output functions as though it were an out parameter, which is a little curious, but all names have the form of expressions, so there is no syntactic irregularity, even though normally functions would not be permitted out parameters.  The third argument is the list of input operands.  It is either a single Asm_Input attribute reference, or a list of such references enclosed in parentheses (technically an array aggregate of such references).

The Asm_Input attribute denotes a function that takes two parameters.  The first is a string, the second is an expression of the type designated by the prefix.  The first (string) argument is required to be a static expression, and is the constraint for the parameter, (e.g. what kind of register is required).  The second argument is the value to be used as the input argument.  The possible values for the constant are the same as those used in the RTL, and are dependent on the configuration file used to built the GCC back end.

If there are no input operands, this argument may either be omitted, or explicitly given as No_Input_Operands.  The fourth argument, not present in the above example, is a list of register names, called the clobber argument.  This argument, if given, must be a static string expression, and is a space or comma separated list of names of registers that must be considered destroyed as a result of the Asm call.  If this argument is the null string (the default value), then the code generator assumes that no additional registers are destroyed.

The fifth argument, not present in the above example, called the volatile argument, is by default False.  It can be set to the literal value True to indicate to the code generator that all optimizations with respect to the instruction specified should be suppressed, and that in particular, for an instruction that has outputs, the instruction will still be generated, even if none of the outputs are used.  See the full description in the GCC manual for further details.

The Asm subprograms may be used in two ways.  First the procedure forms can be used anywhere a procedure call would be valid, and correspond to what the RM calls ``intrinsic'' routines.  Such calls can be used to intersperse machine instructions with other Ada statements. Second, the function forms, which return a dummy value of the limited private type Asm_Insn, can be used in code statements, and indeed this is the only context where such calls are allowed.  Code statements appear as aggregates of the form:

     Asm_Insn'(Asm (...));
     Asm_Insn'(Asm_Volatile (...));

In accordance with RM rules, such code statements are allowed only within subprograms whose entire body consists of such statements.  It is not permissible to intermix such statements with other Ada statements.

Typically the form using intrinsic procedure calls is more convenient and more flexible.  The code statement form is provided to meet the RM suggestion that such a facility should be made available.  The following is the exact syntax of the call to Asm. As usual, if named notation is used, the arguments may be given in arbitrary order, following the normal rules for use of positional and named arguments)

     ASM_CALL ::= Asm (
                      [Template =>] static_string_EXPRESSION
                    [,[Outputs  =>] OUTPUT_OPERAND_LIST      ]
                    [,[Inputs   =>] INPUT_OPERAND_LIST       ]
                    [,[Clobber  =>] static_string_EXPRESSION ]
                    [,[Volatile =>] static_boolean_EXPRESSION] )

     OUTPUT_OPERAND_LIST ::=
       [PREFIX.]No_Output_Operands
     | OUTPUT_OPERAND_ATTRIBUTE
     | (OUTPUT_OPERAND_ATTRIBUTE {,OUTPUT_OPERAND_ATTRIBUTE})

     OUTPUT_OPERAND_ATTRIBUTE ::=
       SUBTYPE_MARK'Asm_Output (static_string_EXPRESSION, NAME)

     INPUT_OPERAND_LIST ::=
       [PREFIX.]No_Input_Operands
     | INPUT_OPERAND_ATTRIBUTE
     | (INPUT_OPERAND_ATTRIBUTE {,INPUT_OPERAND_ATTRIBUTE})

     INPUT_OPERAND_ATTRIBUTE ::=
       SUBTYPE_MARK'Asm_Input (static_string_EXPRESSION, EXPRESSION)

The identifiers No_Input_Operands and No_Output_Operands are declared in the package Machine_Code and must be referenced according to normal visibility rules. In particular if there is no use clause for this package, then appropriate package name qualification is required.

GNAT Implementation of Tasking
==============================

This chapter outlines the basic GNAT approach to tasking (in particular, a multi-layered library for portability) and discusses issues related to compliance with the Real-Time Systems Annex.

Mapping Ada Tasks onto the Underlying Kernel Threads
----------------------------------------------------

GNAT's run-time support comprises two layers:

  * GNARL (GNAT Run-time Layer)

  * GNULL (GNAT Low-level Library)

In GNAT, Ada's tasking services rely on a platform and OS independent layer known as GNARL.  This code is responsible for implementing the correct semantics of Ada's task creation, rendezvous, protected operations etc.

GNARL decomposes Ada's tasking semantics into simpler lower level operations such as create a thread, set the priority of a thread, yield, create a lock, lock/unlock, etc.  The spec for these low-level operations constitutes GNULLI, the GNULL Interface.  This interface is directly inspired from the POSIX real-time API.

If the underlying executive or OS implements the POSIX standard faithfully, the GNULL Interface maps as is to the services offered by the underlying kernel.  Otherwise, some target dependent glue code maps the services offered by the underlying kernel to the semantics expected by GNARL.

Whatever the underlying OS (VxWorks, UNIX, OS/2, Windows NT, etc.) the key point is that each Ada task is mapped on a thread in the underlying kernel.  For example, in the case of VxWorks, one Ada task = one VxWorks task.

In addition Ada task priorities map onto the underlying thread priorities. Mapping Ada tasks onto the underlying kernel threads has several advantages:

  * The underlying scheduler is used to schedule the Ada tasks.  This makes Ada tasks as efficient as kernel threads from a scheduling standpoint.

  * Interaction with code written in C containing threads is eased since at the lowest level Ada tasks and C threads map onto the same underlying kernel concept.

  * When an Ada task is blocked during I/O the remaining Ada tasks are able to proceed.

  * On multiprocessor systems Ada tasks can execute in parallel.

Some threads libraries offer a mechanism to fork a new process, with the child process duplicating the threads from the parent. GNAT does not support this functionality when the parent contains more than one task.

Ensuring Compliance with the Real-Time Annex
--------------------------------------------

Although mapping Ada tasks onto the underlying threads has significant advantages, it does create some complications when it comes to respecting the scheduling semantics specified in the real-time annex (Annex D).

For instance the Annex D requirement for the FIFO_Within_Priorities scheduling policy states:

     When the active priority of a ready task that is not running changes, or the setting of its base priority takes effect, the task is removed from the ready queue for its old active priority and is added at the tail of the ready queue for its new active priority, except in the case where the active priority is lowered due to the loss of inherited priority, in which case the task is added at the head of the ready queue for its new active priority.

While most kernels do put tasks at the end of the priority queue when a task changes its priority, (which respects the main FIFO_Within_Priorities requirement), almost none keep a thread at the beginning of its priority queue when its priority drops from the loss of inherited priority.

As a result most vendors have provided incomplete Annex D implementations.

The GNAT run-time, has a nice cooperative solution to this problem which ensures that accurate FIFO_Within_Priorities semantics are respected.

The principle is as follows.  When an Ada task T is about to start running, it checks whether some other Ada task R with the same priority as T has been suspended due to the loss of priority inheritance.  If this is the case, T yields and is placed at the end of its priority queue.  When R arrives at the front of the queue it executes.

Note that this simple scheme preserves the relative order of the tasks that were ready to execute in the priority queue where R has been placed at the end.

GNAT Implementation of Shared Passive Packages
==============================================

GNAT fully implements the pragma Shared_Passive for the purpose of designating shared passive packages. This allows the use of passive partitions in the context described in the Ada Reference Manual; i.e. for communication between separate partitions of a distributed application using the features in Annex E.

However, the implementation approach used by GNAT provides for more extensive usage as follows:

Communication between separate programs

     This allows separate programs to access the data in passive partitions, using protected objects for synchronization where needed. The only requirement is that the two programs have a common shared file system. It is even possible for programs running on different machines with different architectures (e.g. different endianness) to communicate via the data in a passive partition.

Persistence between program runs

     The data in a passive package can persist from one run of a program to another, so that a later program sees the final values stored by a previous run of the same program.

The implementation approach used is to store the data in files. A separate stream file is created for each object in the package, and an access to an object causes the corresponding file to be read or written.

The environment variable SHARED_MEMORY_DIRECTORY should be set to the directory to be used for these files. The files in this directory have names that correspond to their fully qualified names. For example, if we have the package

     package X is
       pragma Shared_Passive (X);
       Y : Integer;
       Z : Float;
     end X;

and the environment variable is set to /stemp/, then the files created will have the names:

     /stemp/x.y
     /stemp/x.z

These files are created when a value is initially written to the object, and the files are retained until manually deleted. This provides the persistence semantics. If no file exists, it means that no partition has assigned a value to the variable; in this case the initial value declared in the package will be used. This model ensures that there are no issues in synchronizing the elaboration process, since elaboration of passive packages elaborates the initial values, but does not create the files.

The files are written using normal Stream_IO access. If you want to be able to communicate between programs or partitions running on different architectures, then you should use the XDR versions of the stream attribute routines, since these are architecture independent.

If active synchronization is required for access to the variables in the shared passive package, then as described in the Ada Reference Manual, the package may contain protected objects used for this purpose. In this case a lock file (whose name is ___lock (three underscores) is created in the shared memory directory. This is used to provide the required locking semantics for proper protected object synchronization.

As of January 2003, GNAT supports shared passive packages on all platforms except for OpenVMS.

Code Generation for Array Aggregates
====================================

Aggregate have a rich syntax and allow the user to specify the values of complex data structures by means of a single construct.  As a result, the code generated for aggregates can be quite complex and involve loops, case statements and multiple assignments.  In the simplest cases, however, the compiler will recognize aggregates whose components and constraints are fully static, and in those cases the compiler will generate little or no executable code.  The following is an outline of the code that GNAT generates for various aggregate constructs.  For further details, the user will find it useful to examine the output produced by the -gnatG flag to see the expanded source that is input to the code generator.  The user will also want to examine the assembly code generated at various levels of optimization.

The code generated for aggregates depends on the context, the component values, and the type.  In the context of an object declaration the code generated is generally simpler than in the case of an assignment.  As a general rule, static component values and static subtypes also lead to simpler code.

Static constant aggregates with static bounds
---------------------------------------------

For the declarations:

         type One_Dim is array (1..10) of integer;
         ar0 : constant One_Dim := ( 1, 2, 3, 4, 5, 6, 7, 8, 9, 0);

GNAT generates no executable code: the constant ar0 is placed in static memory. The same is true for constant aggregates with named associations:

         Cr1 : constant One_Dim := (4 => 16, 2 => 4, 3 => 9, 1=> 1);
         Cr3 : constant One_Dim := (others => 7777);

The same is true for multidimensional constant arrays such as:

         type two_dim is array (1..3, 1..3) of integer;
         Unit : constant two_dim := ( (1,0,0), (0,1,0), (0,0,1));

The same is true for arrays of one-dimensional arrays: the following are static:

     type ar1b  is array (1..3) of boolean;
     type ar_ar is array (1..3) of ar1b;
     None  : constant ar1b := (others => false);     --  fully static
     None2 : constant ar_ar := (1..3 => None);       --  fully static

However, for multidimensional aggregates with named associations, GNAT will generate assignments and loops, even if all associations are static.  The following two declarations generate a loop for the first dimension, and individual component assignments for the second dimension:

     Zero1: constant two_dim := (1..3 => (1..3 => 0));
     Zero2: constant two_dim := (others => (others => 0));

Constant aggregates with an unconstrained nominal types
-------------------------------------------------------

In such cases the aggregate itself establishes the subtype, so that associations with others cannot be used.  GNAT determines the bounds for the actual subtype of the aggregate, and allocates the aggregate statically as well.  No code is generated for the following:

         type One_Unc is array (natural range <>) of integer;
         Cr_Unc : constant One_Unc := (12,24,36);

Aggregates with static bounds
-----------------------------

In all previous examples the aggregate was the initial (and immutable) value of a constant.  If the aggregate initializes a variable, then code is generated for it as a combination of individual assignments and loops over the target object.  The declarations

            Cr_Var1 : One_Dim := (2, 5, 7, 11);
            Cr_Var2 : One_Dim := (others > -1);

generate the equivalent of

            Cr_Var1 (1) := 2;
            Cr_Var1 (2) := 3;
            Cr_Var1 (3) := 5;
            Cr_Var1 (4) := 11;

            for I in Cr_Var2'range loop
               Cr_Var2 (I) := =-1;
            end loop;

Aggregates with non-static bounds
---------------------------------

If the bounds of the aggregate are not statically compatible with the bounds of the nominal subtype  of the target, then constraint checks have to be generated on the bounds.  For a multidimensional array, constraint checks may have to be applied to sub-arrays individually, if they do not have statically compatible subtypes.

Aggregates in assignment statements
-----------------------------------

In general, aggregate assignment requires the construction of a temporary, and a copy from the temporary to the target of the assignment.  This is because it is not always possible to convert the assignment into a series of individual component assignments.  For example, consider the simple case:

             A := (A(2), A(1));

This cannot be converted into:

             A(1) := A(2);
             A(2) := A(1);

So the aggregate has to be built first in a separate location, and then copied into the target.  GNAT recognizes simple cases where this intermediate step is not required, and the assignments can be performed in place, directly into the target.  The following sufficient criteria are applied:

  * The bounds of the aggregate are static, and the associations are static.

  * The components of the aggregate are static constants, names of simple variables that are not renamings, or expressions not involving indexed components whose operands obey these rules.

If any of these conditions are violated, the aggregate will be built in a temporary (created either by the front-end or the code generator) and then that temporary will be copied onto the target.

The Size of Discriminated Records with Default Discriminants
============================================================

If a discriminated type T has discriminants with default values, it is possible to declare an object of this type without providing an explicit constraint:

     type Size is range 1..100;

     type Rec (D : Size := 15) is record
        Name : String (1..D);
     end T;

     Word : Rec;

Such an object is said to be unconstrained. The discriminant of the object can be modified by a full assignment to the object, as long as it preserves the relation between the value of the discriminant, and the value of the components that depend on it:

     Word := (3, "yes");

     Word := (5, "maybe");

     Word := (5, "no"); -- raises Constraint_Error

In order to support this behavior efficiently, an unconstrained object is given the maximum size that any value of the type requires. In the case above, Word has storage for the discriminant and for a String of length 100. It is important to note that unconstrained objects do not require dynamic allocation. It would be an improper implementation to place on the heap those components whose size depends on discriminants. (This improper implementation was used by some Ada83 compilers, where the Name component above would have been stored as a pointer to a dynamic string). Following the principle that dynamic storage management should never be introduced implicitly, an Ada95 compiler should reserve the full size for an unconstrained declared object, and place it on the stack.

This maximum size approach has been a source of surprise to some users, who expect the default values of the discriminants to determine the size reserved for an unconstrained object: ``If the default is 15, why should the object occupy a larger size?'' The answer, of course, is that the discriminant may be later modified, and its full range of values must be taken into account. This is why the declaration:

     type Rec (D : Positive := 15) is record
        Name : String (1..D);
     end record;

     Too_Large : Rec;

is flagged by the compiler with a warning: an attempt to create Too_Large will raise Storage_Error, because the required size includes Positive'Last bytes. As the first example indicates, the proper approach is to declare an index type of ``reasonable'' range so that unconstrained objects are not too large.

One final wrinkle: if the object is declared to be aliased, or if it is created in the heap by means of an allocator, then it is not unconstrained: it is constrained by the default values of the discriminants, and those values cannot be modified by full assignment. This is because in the presence of aliasing all views of the object (which may be manipulated by different tasks, say) must be consistent, so it is imperative that the object, once created, remain invariant.

Project File Reference
**********************

This chapter describes the syntax and semantics of project files. Project files specify the options to be used when building a system. Project files can specify global settings for all tools, as well as tool-specific settings. See the chapter on project files in the GNAT Users guide for examples of use.

Reserved Words
==============

All Ada95 reserved words are reserved in project files, and cannot be used as variable names or project names. In addition, the following are also reserved in project files:

  * extends

  * external

  * project

Lexical Elements
================

Rules for identifiers are the same as in Ada95. Identifiers are case-insensitive.  Strings are case sensitive, except where noted. Comments have the same form as in Ada95.

Syntax:

     simple_name ::=
       identifier

     name ::=
       simple_name {. simple_name}

Declarations
============

Declarations introduce new entities that denote types, variables, attributes, and packages. Some declarations can only appear immediately within a project declaration. Others can appear within a project or within a package.

Syntax:

     declarative_item ::=
       simple_declarative_item |
       typed_string_declaration |
       package_declaration

     simple_declarative_item ::=
       variable_declaration |
       typed_variable_declaration |
       attribute_declaration |
       case_construction |
       empty_declaration

Empty declarations
==================

     empty_declaration ::=
       null ;

An empty declaration is allowed anywhere a declaration is allowed. It has no effect.

Typed string declarations
=========================

Typed strings are sequences of string literals. Typed strings are the only named types in project files. They are used in case constructions, where they provide support for conditional attribute definitions.

Syntax:

     typed_string_declaration ::=
       type <typed_string_>_simple_name is
        ( string_literal {, string_literal} );

A typed string declaration can only appear immediately within a project declaration.

All the string literals in a typed string declaration must be distinct.

Variables
=========

Variables denote values, and appear as constituents of expressions.

     typed_variable_declaration ::=
       <typed_variable_>simple_name : <typed_string_>name :=  string_expression ;

     variable_declaration ::=
       <variable_>simple_name := expression;

The elaboration of a variable declaration introduces the variable and assigns to it the value of the expression. The name of the variable is available after the assignment symbol.

A typed_variable can only be declare once.

a non typed variable can be declared multiple times.

Before the completion of its first declaration, the value of variable is the null string.

Expressions
===========

An expression is a formula that defines a computation or retrieval of a value. In a project file the value of an expression is either a string or a list of strings. A string value in an expression is either a literal, the current value of a variable, an external value, an attribute reference, or a concatenation operation.

Syntax:

     expression ::=
       term {& term}

     term ::=
       string_literal |
       string_list |
       <variable_>name |
       external_value |
       attribute_reference

     string_literal ::=
       (same as Ada)

     string_list ::=
       ( <string_>expression { , <string_>expression } )

Concatenation
-------------

The following concatenation functions are defined:

       function "&" (X : String;      Y : String)      return String;
       function "&" (X : String_List; Y : String)      return String_List;
       function "&" (X : String_List; Y : String_List) return String_List;

Attributes
==========

An attribute declaration defines a property of a project or package. This property can later be queried by means of an attribute reference. Attribute values are strings or string lists.

Some attributes are associative arrays. These attributes are mappings whose domain is a set of strings. These attributes are declared one association at a time, by specifying a point in the domain and the corresponding image of the attribute. They may also be declared as a full associative array, getting the same associations as the corresponding attribute in an imported or extended project.

Attributes that are not associative arrays are called simple attributes.

Syntax:

     attribute_declaration ::=
       full_associative_array_declaration |
       for attribute_designator use expression ;

     full_associative_array_declaration ::=
       for <associative_array_attribute_>simple_name use
       <project_>simple_name [ . <package_>simple_Name ] ' <attribute_>simple_name ;

     attribute_designator ::=
       <simple_attribute_>simple_name |
       <associative_array_attribute_>simple_name ( string_literal )

Some attributes are project-specific, and can only appear immediately within a project declaration. Others are package-specific, and can only appear within the proper package.

The expression in an attribute definition must be a string or a string_list. The string literal appearing in the attribute_designator of an associative array attribute is case-insensitive.

Project Attributes
==================

The following attributes apply to a project. All of them are simple attributes.

Object_Dir
     Expression must be a path name. The attribute defines the directory in which the object files created by the build are to be placed. If not specified, object files are placed in the project directory.

Exec_Dir
     Expression must be a path name. The attribute defines the directory in which the executables created by the build are to be placed. If not specified, executables are placed in the object directory.

Source_Dirs
     Expression must be a list of path names. The attribute defines the directories in which the source files for the project are to be found. If not specified, source files are found in the project directory.

Source_Files
     Expression must be a list of file names. The attribute defines the individual files, in the project directory, which are to be used as sources for the project. File names are path_names that contain no directory information. If the project has no sources the attribute must be declared explicitly with an empty list.

Source_List_File
     Expression must a single path name. The attribute defines a text file that contains a list of source file names to be used as sources for the project

Library_Dir
     Expression must be a path name. The attribute defines the directory in which a  library is to be built.  The directory must exist, must be distinct from the project's object directory, and must be writable.

Library_Name
     Expression must be a string that is a legal file name, without extension. The attribute defines a string that is used to generate the name of the library to be built by the project.

Library_Kind
     Argument must be a string value that must be one of the following "static", "dynamic" or "relocatable". This string is case-insensitive. If this attribute is not specified, the library is a static library. Otherwise, the library may be dynamic or relocatable. This distinction is operating-system dependent.

Library_Version
     Expression must be a string value whose interpretation is platform dependent. On UNIX, it is used only for dynamic/relocatable libraries as the internal name of the library (the "soname"). If the library file name (built from the Library_Name) is different from the Library_Version, then the library file will be a symbolic link to the actual file whose name will be Library_Version.

Library_Interface
     Expression must be a string list. Each element of the string list must designate a unit of the project. If this attribute is present in a Library Project File, then the project file is a Stand-alone Library_Project_File.

Library_Auto_Init
     Expression must be a single string "true" or "false", case-insensitive. If this attribute is present in a Stand-alone Library Project File, it indicates if initialization is automatic when the dynamic library is loaded.

Library_Options
     Expression must be a string list. Indicates additional switches that are to be used when building a shared library.

Library_GCC
     Expression must be a single string. Designates an alternative to "gcc" for building shared libraries.

Library_Src_Dir
     Expression must be a path name. The attribute defines the directory in which the sources of the interfaces of a Stand-alone Library will be copied.  The directory must exist, must be distinct from the project's object directory and source directories, and must be writable.

Main
     Expression must be a list of strings that are legal file names. These file names designate existing compilation units in the source directory that are legal main subprograms.

     When a project file is elaborated, as part of the execution of a gnatmake command, one or several executables are built and placed in the Exec_Dir. If the gnatmake command does not include explicit file names, the executables that are built correspond to the files specified by this attribute.

Main_Language
     This is a simple attribute. Its value is a string that specifies the language of the main program.

Languages
     Expression must be a string list. Each string designates a programming language that is known to GNAT. The strings are case-insensitive.

Locally_Removed_Files
     This attribute is legal only in a project file that extends another. Expression must be a list of strings that are legal file names. Each file name must designate a source that would normally be inherited by the current project file. It cannot designate an immediate source that is not inherited. Each of the source files in the list are not considered to be sources of the project file: they are not inherited.

Attribute References
====================

Attribute references are used to retrieve the value of previously defined attribute for a package or project. Syntax:

     attribute_reference ::=
       attribute_prefix ' <simple_attribute_>simple_name [ ( string_literal ) ]

     attribute_prefix ::=
       project |
       <project_simple_name | package_identifier |
       <project_>simple_name . package_identifier

If an attribute has not been specified for a given package or project, its value is the null string or the empty list.

External Values
===============

An external value is an expression whose value is obtained from the command that invoked the processing of the current project file (typically a gnatmake command).

Syntax:

     external_value ::=
       external ( string_literal [, string_literal] )

The first string_literal is the string to be used on the command line or in the environment to specify the external value. The second string_literal, if present, is the default to use if there is no specification for this external value either on the command line or in the environment.

Case Construction
=================

A case construction supports attribute declarations that depend on the value of a previously declared variable.

Syntax:

     case_construction ::=
       case <typed_variable_>name is
         {case_item}
       end case ;

     case_item ::=
       when discrete_choice_list =>
         {case_construction | attribute_declaration | empty_declaration}

     discrete_choice_list ::=
       string_literal {| string_literal} |
         others

All choices in a choice list must be distinct. The choice lists of two distinct alternatives must be disjoint. Unlike Ada, the choice lists of all alternatives do not need to include all values of the type. An others choice must appear last in the list of alternatives.

Packages
========

A package provides a grouping of variable declarations and attribute declarations to be used when invoking various GNAT tools. The name of the package indicates the tool(s) to which it applies. Syntax:

     package_declaration ::=
       package_specification | package_renaming

     package_specification ::=
       package package_identifier is
         {simple_declarative_item}
       end package_identifier ;

     package_identifier ::=
       Naming | Builder | Compiler | Binder |
       Linker | Finder  | Cross_Reference |
       gnatls | IDE     | Pretty_Printer

Package Naming
--------------

The attributes of a Naming package specifies the naming conventions that apply to the source files in a project. When invoking other GNAT tools, they will use the sources in the source directories that satisfy these naming conventions.

The following attributes apply to a Naming package:

Casing
     This is a simple attribute whose value is a string. Legal values of this string are "lowercase", "uppercase" or "mixedcase". These strings are themselves case insensitive.

     If Casing is not specified, then the default is "lowercase".

Dot_Replacement
     This is a simple attribute whose string value satisfies the following requirements:

       * It must not be empty

       * It cannot start or end with an alphanumeric character

       * It cannot be a single underscore

       * It cannot start with an underscore followed by an alphanumeric

       * It cannot contain a dot '.' if longer than one character

     If Dot_Replacement is not specified, then the default is "-".

Spec_Suffix
     This is an associative array attribute, defined on language names, whose image is a string that must satisfy the following conditions:

       * It must not be empty

       * It cannot start with an alphanumeric character

       * It cannot start with an underscore followed by an alphanumeric character

     For Ada, the attribute denotes the suffix used in file names that contain library unit declarations, that is to say units that are package and subprogram declarations. If Spec_Suffix ("Ada") is not specified, then the default is ".ads".

     For C and C++, the attribute denotes the suffix used in file names that contain prototypes.

Body_Suffix
     This is an associative array attribute defined on language names, whose image is a string that must satisfy the following conditions:

       * It must not be empty

       * It cannot start with an alphanumeric character

       * It cannot start with an underscore followed by an alphanumeric character

       * It cannot be a suffix of Spec_Suffix

     For Ada, the attribute denotes the suffix used in file names that contain library bodies, that is to say units that are package and subprogram bodies. If Body_Suffix ("Ada") is not specified, then the default is ".adb".

     For C and C++, the attribute denotes the suffix used in file names that contain source code.

Separate_Suffix
     This is a simple attribute whose value satisfies the same conditions as Body_Suffix.

     This attribute is specific to Ada. It denotes the suffix used in file names that contain separate bodies. If it is not specified, then it defaults to same value as Body_Suffix ("Ada").

Spec
     This is an associative array attribute, specific to Ada, defined over compilation unit names. The image is a string that is the name of the file that contains that library unit. The file name is case sensitive if the conventions of the host operating system require it.

Body
     This is an associative array attribute, specific to Ada, defined over compilation unit names. The image is a string that is the name of the file that contains the library unit body for the named unit. The file name is case sensitive if the conventions of the host operating system require it.

Specification_Exceptions
     This is an associative array attribute defined on language names, whose value is a list of strings.

     This attribute is not significant for Ada.

     For C and C++, each string in the list denotes the name of a file that contains prototypes, but whose suffix is not necessarily the Spec_Suffix for the language.

Implementation_Exceptions
     This is an associative array attribute defined on language names, whose value is a list of strings.

     This attribute is not significant for Ada.

     For C and C++, each string in the list denotes the name of a file that contains source code, but whose suffix is not necessarily the Body_Suffix for the language.

The following attributes of package Naming are obsolescent. They are kept as synonyms of other attributes for compatibility with previous versions of the Project Manager.

Specification_Suffix
     This is a synonym of Spec_Suffix.

Implementation_Suffix
     This is a synonym of Body_Suffix.

Specification
     This is a synonym of Spec.

Implementation
     This is a synonym of Body.

package Compiler
----------------

The attributes of the Compiler package specify the compilation options to be used by the underlying compiler.

Default_Switches
     This is an associative array attribute. Its domain is a set of language names. Its range is a string list that specifies the compilation options to be used when compiling a component written in that language, for which no file-specific switches have been specified..

Switches
     This is an associative array attribute. Its domain is a set of file names. Its range is a string list that specifies the compilation options to be used when compiling the named file. If a file is not specified in the Switches attribute, it is compiled with the settings specified by Default_Switches.

Local_Configuration_Pragmas.
     This is a simple attribute, whose value is a path name that designates a file containing configuration pragmas to be used for all invocations of the compiler for immediate sources of the project.

Executable
     This is an associative array attribute. Its domain is a set of main source file names. Its range is a simple string that specifies the executable file name to be used when linking the specified main source. If a main source is not specified in the Executable attribute, the executable file name is deducted from the main source file name.

package Builder
---------------

The attributes of package Builder specify the compilation, binding, and linking options to be used when building an executable for a project. The following attributes apply to package Builder:

Default_Switches
     As above.

Switches
     As above.

Global_Configuration_Pragmas
     This is a simple attribute, whose value is a path name that designates a file that contains configuration pragmas to be used in every build of an executable. If both local and global configuration pragmas are specified, a compilation makes use of both sets.

Executable
     This is an associative array attribute, defined over compilation unit names. The image is a string that is the name of the executable file corresponding to the main source file index. This attribute has no effect if its value is the empty string.

Executable_Suffix
     This is a simple attribute whose value is a suffix to be added to the executables that don't have an attribute Executable specified.

package Gnatls
--------------

The attributes of package Gnatls specify the tool options to be used when invoking the library browser gnatls. The following attributes apply to package Gnatls:

Switches
     As above.

package Binder
--------------

The attributes of package Binder specify the options to be used when invoking the binder in the construction of an executable. The following attributes apply to package Binder:

Default_Switches
     As above.

Switches
     As above.

package Linker
--------------

The attributes of package Linker specify the options to be used when invoking the linker in the construction of an executable. The following attributes apply to package Linker:

Default_Switches
     As above

Switches
     As above.

package Cross_Reference
-----------------------

The attributes of package Cross_Reference specify the tool options to be used when invoking the library tool gnatxref. The following attributes apply to package Cross_Reference:

Default_Switches
     As above.

Switches
     As above.

package Finder
--------------

The attributes of package Finder specify the tool options to be used when invoking the search tool gnatfind. The following attributes apply to package Finder:

Default_Switches
     As above.

Switches
     As above.

package Pretty_Printer
----------------------

The attributes of package Pretty_Printer specify the tool options to be used when invoking the formatting tool gnatpp. The following attributes apply to package Pretty_Printer:

Default_switches
     As above.

Switches
     As above.

package IDE
-----------

The attributes of package IDE specify the options to be used when using an Integrated Development Environment such as GPS.

Remote_Host
     This is a simple attribute. Its value is a string that designates the remote host in a cross-compilation environment, to be used for remote compilation and debugging. This field should not be specified when running on the local machine.

Program_Host
     This is a simple attribute. Its value is a string that specifies the name of IP address of the embedded target in a cross-compilation environment, on which the program should execute.

Communication_Protocol
     This is a simple string attribute. Its value is the name of the protocol to use to communicate with the target in a cross-compilation environment, e.g. "wtx" or "vxworks".

Compiler_Command
     This is an associative array attribute, whose domain is a language name. Its value is  string that denotes the command to be used to invoke the compiler. The value of Compiler_Command ("Ada") is expected to be compatible with gnatmake, in particular in the handling of switches.

Debugger_Command
     This is simple attribute, Its value is a string that specifies the name of the debugger to be used, such as gdb, powerpc-wrs-vxworks-gdb or gdb-4.

Default_Switches
     This is an associative array attribute. Its indexes are the name of the external tools that the GNAT Programming System (GPS) is supporting. Its value is a list of switches to use when invoking that tool.

Gnatlist
     This is a simple attribute.  Its value is a string that specifies the name of the gnatls utility to be used to retrieve information about the predefined path; e.g., "gnatls", "powerpc-wrs-vxworks-gnatls".

VCS_Kind
     This is a simple atribute. Is value is a string used to specify the Version Control System (VCS) to be used for this project, e.g CVS, RCS ClearCase or Perforce.

VCS_File_Check
     This is a simple attribute. Its value is a string that specifies the command used by the VCS to check the validity of a file, either when the user explicitly asks for a check, or as a sanity check before doing the check-in.

VCS_Log_Check
     This is a simple attribute. Its value is a string that specifies the command used by the VCS to check the validity of a log file.

Package Renamings
=================

A package can be defined by a renaming declaration. The new package renames a package declared in a different project file, and has the same attributes as the package it renames. Syntax:

     package_renaming ::==
       package package_identifier renames
            <project_>simple_name.package_identifier ;

The package_identifier of the renamed package must be the same as the package_identifier. The project whose name is the prefix of the renamed package must contain a package declaration with this name. This project must appear in the context_clause of the enclosing project declaration, or be the parent project of the enclosing child project.

Projects
========

A project file specifies a set of rules for constructing a software system. A project file can be self-contained, or depend on other project files. Dependencies are expressed through a context clause that names other projects.

Syntax:

     project ::=
       context_clause project_declaration

     project_declaration ::=
       simple_project_declaration | project_extension

     simple_project_declaration ::=
       project <project_>simple_name is
         {declarative_item}
       end <project_>simple_name;

     context_clause ::=
       {with_clause}

     with_clause ::=
       [limited] with path_name { , path_name } ;

     path_name ::=
        string_literal

A path name denotes a project file. A path name can be absolute or relative. An absolute path name includes a sequence of directories, in the syntax of the host operating system, that identifies uniquely the project file in the file system. A relative path name identifies the project file, relative to the directory that contains the current project, or relative to a directory listed in the environment variable ADA_PROJECT_PATH. Path names are case sensitive if file names in the host operating system are case sensitive.

The syntax of the environment variable ADA_PROJECT_PATH is a list of directory names separated by colons (semicolons on Windows).

A given project name can appear only once in a context_clause.

It is illegal for a project imported by a context clause to refer, directly or indirectly, to the project in which this context clause appears (the dependency graph cannot contain cycles), except when one of the with_clause in the cycle is a limited with.

Project Extensions
==================

A project extension introduces a new project, which inherits the declarations of another project. Syntax:

     project_extension ::=
       project <project_>simple_name  extends path_name is
         {declarative_item}
       end <project_>simple_name;

The project extension declares a child project. The child project inherits all the declarations and all the files of the parent project, These inherited declaration can be overridden in the child project, by means of suitable declarations.

Project File Elaboration
========================

A project file is processed as part of the invocation of a gnat tool that uses the project option. Elaboration of the process file consists in the sequential elaboration of all its declarations. The computed values of attributes and variables in the project are then used to establish the environment in which the gnat tool will execute.

Obsolescent Features
********************

This chapter describes features that are provided by GNAT, but are considered obsolescent since there are preferred ways of achieving the same effect. These features are provided solely for historical compatibility purposes.

pragma No_Run_Time
==================

The pragma No_Run_Time is used to achieve an affect similar to the use of the "Zero Foot Print" configurable run time, but without requiring a specially configured run time. The result of using this pragma, which must be used for all units in a partition, is to restrict the use of any language features requiring run-time support code. The preferred usage is to use an appropriately configured run-time that includes just those features that are to be made accessible.

pragma Ravenscar
================

The pragma Ravenscar has exactly the same effect as pragma Profile (Ravenscar). The latter usage is preferred since it is part of the new Ada 2005 standard.

pragma Restricted_Run_Time
==========================

The pragma Restricted_Run_Time has exactly the same effect as pragma Profile (Restricted). The latter usage is preferred since the Ada 2005 pragma Profile is intended for this kind of implementation dependent addition.

     [file fdl.texi not included]